Heat the oil in your slow cooker if it allows for searing or in a separate pan. Brown the meat over medium-high heat, in batches, until nicely browned. Use a slotted spoon to transfer to a bowl.
Add a splash more oil if needed and reduce the heat. Sauté the onion and mushrooms for 5-7 minutes, stirring, or until softened.
Stir in the garlic then the sugar, salt, pepper, tomato paste and cranberry sauce. (If you have been using a separate pan transfer everything to your slow cooker now).
Add the meat, chestnuts, cranberries, beef broth, wine and Worcester Sauce and stir to combine.
Cover and cook on HIGH for 4 -5 hours or LOW for 7 -8 hours or until the venison is tender.
Check the seasoning and add salt and pepper if needed.
Serve over mashed potatoes sprinkled with fresh thyme.
To cook in a Dutch Oven, follow the steps to brown and sauté on the hob. Double the amount of beef broth and red wine then transfer to the oven, preheated at 160°C (325°F), for 1 1/2 - 2 hours or until the venison is tender.
Keep any leftovers in the fridge for up to three days or freeze for up to three months.