Preheat your oven to 350°F.
Prepare the mushrooms: Place them on a cutting board or cookie sheet and inspect for dirt or blemishes, using a paring knife to carefully scrape or cut away any impurities.
Slice or cut the clean mushrooms into large chunks. Aim for uniformity but keep pieces larger than your other vegetables. If they’re small (like a garlic clove) leave them whole.
In an oven-safe Dutch oven, melt the butter over medium heat. Once it starts to bubble and brown slightly, add the mushrooms with a large pinch of salt and freshly ground black pepper.
Sauté the mushrooms until golden brown. Add the leeks, carrots, onion, fennel, and garlic, cooking just until they begin to soften.
Add the wine and reduce by half.
Stir in the tomato paste and vegetable stock until well combined.
Add another large pinch of salt along with the bay leaves. Place the tomatoes on top, allowing them to fl oat and roast—do not stir them in.
Transfer the Dutch oven to the preheated oven and cook for 2 hours. Check every 30 minutes, adding a bit more stock if needed.
After 2 hours, remove the Dutch oven and place it on a burner over low heat. Check the mushrooms with a fork; they should be soft and uniformly orange.
Reduce any remaining liquid until the stew thickens, then stir in the heavy cream. Season with salt and pepper to taste.
Serve over polenta or accompany with crusty bread for a fantastic fall meal.