Start by pan frying the sausages in a hot pan, turning them a few times until they are nicely browned on all sides then set aside. They don’t have to be cooked all the way through at this stage.
Add the butter and oil in the same pan and lower the heat. Pan fry the onions, stirring over low heat for about 10 minutes until they have softened.
Stir in the flour and gradually add the beef stock to make a gravy. Add the onion chutney, tomato paste and mustard and bring to a simmer. Stir in the wine and continue to cook until the gravy reduces slightly.
Slice the sausages into bite sized pieces and stir into the gravy together with the herbs. Taste and add salt and pepper then transfer to your pie dish while you prepare the topping.
Preheat the oven to 200C(400F). Boil the potatoes in salted water, drain and leave for 5 minutes. Add warm milk, butter, minced garlic and mustard and mash well. Taste and season with salt and pepper.
Combine the egg yolks and sour cream and stir into the mash.
Spoon the mash over the sausages in your pie dish and swirl the surface with the back of your spoon. Add grated nutmeg and Parmesan and place your pie dish on a rimmed tray (to catch any spills). Bake for 35-40 minutes or until the mash is golden and the filling is bubbling at the edges of the pie.
Cool slightly before serving with some green beans, wilted spinach or your favourite vegetables.
Video
Notes
If you can't find caramelised onion chutney then use a tablespoon of cranberry sauce or seedless raspberry jam to add a touch of sweetness.