French Onion Mashed Potato Gratin
, Updated Nov 23, 2025
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French Onion Mashed Potato Gratin — meet THE ultimate comfort food side! Creamy mashed potatoes topped with jammy caramelized onions and thinly sliced scalloped potatoes under a blanket of grated cheese. This impressive and indulgent side dish, inspired by Chef John’s mashed potato au gratin recipe, is sure to be a hit with everyone.
This Potato Gratin is what happens when mashed potatoes, French onion soup and scalloped potatoes have a scandalous love affair. It starts with a creamy mashed potato base, topped with a layer of rich caramelized onions and finishes with thin potato slices and lots of cheese. This potato bake is not for the faint of heart – but it makes for an incredible side dish that’s sure to steal the show at your holiday dinner!
What Does “Au Gratin” Mean?
The term “au gratin” comes from the French verb gratter, meaning to grate. In culinary terms, it refers to any dish that’s topped with cheese or breadcrumbs (or both!) and baked until the surface turns golden and crisp.
The most well-knows is probably Gratin Dauphinois, a traditional French dish from the Dauphiné region made with thinly sliced potatoes baked in cream, milk, and sometimes cheese, creating a rich, silky interior with a golden crust.
This Mashed Potato Gratin borrows from that same French tradition — layers of creamy potatoes, cheese, and a golden baked top — but adds a twist with a mashed potato base and a touch of French onion flavor. The result? A spectacular potato side dish that’s classic and comforting, yet totally new.
Here’s What You’ll Need
- For the caramelized onions: Loads of thinly sliced onions, garlic cloves, thyme, butter and olive oil, brown sugar and Worcestershire sauce. I like to add brandy or Marsala wine for extra flavor but you can use water or stock if preferred.
- For the mashed potatoes: Use Russet potatoes or Yukon Gold (Maris Piper in the UK). You will also need unsalted butter, whole milk, grated Gruyere or cheddar cheese, salt, black pepper, nutmeg and pinch cayenne pepper.
- For the au gratin potatoes: Thinly sliced potatoes, melted butter, grated Parmesan or Pecorino Romano and grated cheddar cheese (this dish is unashamedly unrestrained!).
How to Make my French Onion Mashed Potatoes Au Gratin
Make the caramelized onions
- Heat butter and olive oil in a large skillet. Add the sliced onions, whole garlic cloves, salt, pepper, Worcestershire sauce and brown sugar. Cook over medium-high heat, stirring frequently, until the onions start to soften and turn golden.
- Add a tiny splash of Marsala, brandy or stock to the pan whenever the onions start to stick and continue to cook until they turn a deep brown color (30-40 minutes).
- Take off the heat and pick out the whole garlic cloves. Squeeze the garlic out, mash into a paste and stir back into the onions.
Prepare the mashed potatoes
- Cut six of the potatoes into quarters and remaining four in half (those will be used for the topping). Place in a large pot, cover with cold water, add salt and bring to the boil. Set a timer for 8 minutes once the water starts boiling – at this point you need to remove the halved potatoes using tongs and set them aside on paper towels to cool.
- Continue cooking the rest of the potatoes until fork tender (about 13-15 minutes total). Drain and let them steam dry in the colander then return to the pot. Add some of the warm milk and butter and use a potato masher to mash them roughly.
- Add remaining milk, butter and grated cheese and mash until smooth. Have a taste and season with salt, pepper, cayenne and grated nutmeg. Set aside.
Assemble the mashed potato gratin
- Preheat the oven to 400°F (200°C). Slice the potatoes for the topping :use a sharp knife to slice the parboiled potatoes into thin half moons – they will be almost entirely cooked through, so handle them with care so they don’t fall apart.
- Brush a baking dish with melted butter. Spoon the mashed potatoes into the dish and level. Top with the caramelized onions, spreading to cover the mash.
- Lay the sliced potatoes over the top, overlapping them slightly (but don’t crowd them!). Brush with melted butter, season with salt and pepper and top with grated cheddar and Parmesan dish.
- Place the dish on a rimmed baking sheet to catch any spills and bake for 40–60 minutes or until the potatoes are fully tender when pierced with a knife and the topping is bubbly and golden. Cool for 10-15 minutes (at least!) before serving.
- You can also bake this in the air fryer (but check the dish fits your air fryer basket first). Preheat to 375°C (190°C) and cook for 30-40 minutes, checking after 20 minutes to see if the potatoes are browning too quickly. You can reduce the temperature to 350°C (180°C) if that’s the case.
Recipe Notes and FAQs
- Do I need to parboil the potatoes for the topping? Strictly speaking you don’t…. but whenever I don’t pre-cook them, I inevitably end up with some potatoes not quite cooking through. By giving them a head start we also cut down on the overall cooking time.
- Make sure to slice the potatoes evenly – you don’t need paper thin slices since if they are pre-cooked. If you want to use raw potatoes for the topping, use a mandoline to slice them into very thin crescents.
- Tent the baking dish with foil if the cheesy topping is browning too much – but make sure it is not touching the potatoes otherwise it will stick!
- Can I prep ahead? You can assemble the gratin up to 24 hours in advance. Cover the prepare dish, refrigerate and bake before serving.
- Leftovers and storage: Store for up to 3 days in an airtight container in the refrigerator and warm in the oven at 350°F (180°C) until piping hot.
Hope you fall in love with this Mashed Potato Gratin recipe! Please leave a comment and rating if you try it, follow me on Pinterest for more easy recipes and don’t forget to tag me on @Instagram or TikTok so I can see your creations.
Mashed Potato Gratin
This Mashed Potato Gratin is comfort food dressed up fancy for the holidays — buttery mash, caramelized onions and crisp, cheesy potatoes baked to perfection. A spectacular potato side dish that’s sure to become a family favorite.
Video
Ingredients
For the caramelized onions
- 4 large onions
- 1 tbsp olive oil
- 1 tbsp unsalted butter
- 6 cloves garlic , peel on, whole
- 2 tsp fresh thyme
- ½ tsp salt
- ½ tsp black pepper
- 1 tbsp brown sugar
- 1 tbsp Worcestershire sauce
- ⅓ cup (80ml) Marsala or chicken stock or as needed
For the potatoes
- 6 large russet potatoes , peeled and quartered
- 4 large russet potatoes , peeled and sliced in half
- 2 tsp salt
For the mashed potatoes
- ¾ cup (180ml) whole milk or more, if needed
- 6 tbsp unsalted butter , cubed
- ½ cup (115g) Gruyere or cheddar cheese, grated
- ¾ tsp grated nutmeg , or to taste
- ⅕ tsp cayenne
- salt and black pepper , to taste
For the au gratin topping
- 3 tablespoons melted butter
- 3 tbsp sharp cheddar or gruyere cheese, grated
- ½ cup (50g) parmesan cheese grated
- 2 tsp cream (optional)
- ½ tsp salt and black pepper
Instructions
Prepare the caramelized onions
- Peel the onions, slice in half and then use a sharp knife to cut into half moon shape.4 large onions
- Heat butter and olive oil in a large skillet. Add the sliced onions, whole garlic cloves, salt, pepper, Worcestershire sauce and brown sugar. Cook over medium-high heat, stirring frequently, until the onions start to soften and turn golden.1 tbsp olive oil, 1 tbsp unsalted butter, 6 cloves garlic, 2 tsp fresh thyme, ½ tsp salt, ½ tsp black pepper, 1 tbsp brown sugar, 1 tbsp Worcestershire sauce
- Add a splash of Marsala, brandy or chicken stock to the pan whenever it becomes too dry and the onions start to stick and continue to cook until they turn a deep brown color.⅓ cup (80ml) Marsala or chicken stock
- Take off the heat and pick out the whole garlic cloves. Squeeze the garlic out, mash into a paste and stir back into the onions.
Cook the potatoes
- Cut the 6 large potatoes into quarters and remaining four in half. Place in a large pot, cover with cold water and season with salt. Bring to a boil and cook for 8 minutes – at this point you need to remove the halved potatoes using tongs and set them aside on paper towels to cool.6 large russet potatoes, 4 large russet potatoes, 2 tsp salt
- Continue cooking the rest of the potatoes until fork tender (about 13-15 minutes total).
Make the mashed potatoes
- Drain the cooked quartered potatoes, let them steam dry in the colander then return to the pot.
- Warm the milk in a saucepan or the microwave. Add some of the warm milk and butter and use a potato masher to mash them roughly.¾ cup (180ml) whole milk, 6 tbsp unsalted butter
- Add remaining milk, butter and grated cheese and mash until smooth. Season with the salt, pepper, cayenne and grated nutmeg. Set aside.½ cup (115g) Gruyere, ¾ tsp grated nutmeg, ⅕ tsp cayenne, salt and black pepper
Slice the potatoes for the topping
- Preheat the oven to 400°F (200°C). Use a sharp knife to slice the parboiled potatoes into thin half moons – they will be almost entirely cooked through, so handle them with care so they don't fall apart.
- You can also bake this in the air fryer (but check the dish fits your air fryer basket first). Preheat to 375°C (190°C) and cook for 30-40 minutes, checking after 20 minutes to see if the potatoes are browning too quickly. You can reduce the temperature to 350°C (180°C) if that's the case.
Assemble the mashed potato gratin
- Brush a baking dish with some of the melted butter. Spoon the mashed potatoes into the dish and level. Top with the caramelized onions, spreading them out to cover the mash.3 tablespoons melted butter
- Lay the sliced potatoes over the top in 3 or 4 rows, overlapping them slightly (but don't crowd them!). Brush with melted butter, season with salt and pepper and top with the grated cheese. Drizzle with the cream if using.3 tbsp sharp cheddar, ½ cup (50g) parmesan cheese, 2 tsp cream, ½ tsp salt and black pepper
- Place the dish on a rimmed baking sheet to catch any spills and bake for 40–60 minutes or until the potatoes are fully tender when pierced with a knife and the topping is bubbly and golden. Cool for 10-15 minutes before serving.
Notes
- Do I need to parboil the potatoes for the topping? Strictly speaking you don’t…. but whenever I don’t pre-cook them I inevitably end up with some potatoes not quite cooking through. By giving them a head start we can cut down on the overall cooking time.
- Make sure to slice the potatoes evenly – you don’t need paper thin slices since if they are pre-cooked. If you want to use raw potatoes for the topping, use a mandoline to slice them into very thin crescents.
- Tent the baking dish with foil if the cheesy topping is browning too much – but make sure it is not touching the potatoes otherwise it will stick!
- Can I prep ahead? You can assemble the gratin up to 24 hours in advance, refrigerate and bake before serving.
- Leftovers and storage: Store for up to 3 days in an airtight container in the refrigerator and warm in the oven at 350°F (180°C) until piping hot.
Nutrition
Calories: 643kcal | Carbohydrates: 95g | Protein: 18g | Fat: 24g | Saturated Fat: 14g | Polyunsaturated Fat: 1g | Monounsaturated Fat: 7g | Trans Fat: 1g | Cholesterol: 61mg | Sodium: 1154mg | Potassium: 2115mg | Fiber: 7g | Sugar: 8g | Vitamin A: 728IU | Vitamin C: 33mg | Calcium: 315mg | Iron: 4mg
Nutritional information is always approximate, and will depend on quality of ingredients and serving sizes.
