1. Slice the sausage into thick slices, about 1 inch (2.5 cm) thick, and fry them in butter in a large pot, ideally a cast-iron one, over medium heat for about 2-3 minutes on each side, or until browned.

2. Peel and chop the onion into medium-sized pieces and mince 2-3 cloves of garlic. Add them to the pot and sauté for another minute or two, stirring occasionally, until they're softened and fragrant.

3. Peel and prepare the root vegetables, such as carrots and parsnips, and mushrooms. Chop the root vegetables into bite-sized pieces, about 1/2 inch (1 cm) in size, and cut the mushrooms into wedges. Add them to the pot, seasoning with salt and pepper to taste.

4. Pour in 1/2 cup (120 ml) of white wine and 1 cup (240 ml) of vegetable broth, if using. Let the stew simmer over low heat, covered, for about 15-20 minutes, or until the liquid has reduced slightly and the flavors have melded together.

5. Add 1/2 cup (120 ml) of heavy cream, 1 tablespoon of Dijon mustard or tomato puree, and any desired spices, such as paprika or dried thyme, to the pot. Let the stew simmer, uncovered, for another 5-10 minutes, or until the vegetables are tender but still crisp.

6. Taste and adjust the seasoning as needed. Serve the stew hot, garnished with fresh thyme, alongside boiled rice or potatoes and a side salad.