Preheat your oven to 400°F (200°C).

In a bowl, combine the chopped roasted chestnuts and finely chopped rosemary. Mix well and set aside.

Pat the bison ribeye steaks dry with paper towels. Season both sides with salt and pepper.

Heat olive oil in a large oven-safe skillet over medium-high heat. Once hot, sear the bison ribeye steaks for 3-4 minutes on each side until browned.

Remove the skillet from heat and brush the tops of the steaks with Dijon mustard. Press the chestnut and rosemary mixture on top of the mustard-coated side of the steaks, ensuring an even crust.

Transfer the skillet to the preheated oven and roast the steaks for about 10-12 minutes for medium-rare, or until a meat thermometer reads 130°F (54°C). Adjust the cooking time based on your preferred doneness.

While the steaks are roasting, prepare the wild mushroom gravy. In a separate skillet, melt the butter over medium heat. Add the sliced wild mushrooms and sauté for about 5-7 minutes until they are tender and golden.

Pour in the beef broth and Worcestershire sauce, bringing the mixture to a simmer. Let it cook for about 5 minutes.

Stir in the heavy cream and continue to simmer for another 2-3 minutes until the gravy thickens. Season with salt and pepper to taste.

Once the bison ribeye steaks are done, remove them from the oven and let them rest for 5 minutes before slicing.

Serve the sliced bison ribeye with the velvety wild mushroom gravy drizzled over the top. Garnish with fresh thyme if desired.