Preheat your oven to 375F.
In a large skillet, melt the butter over medium heat. Add the minced shallots and garlic, sauteing them for about 30 seconds until fragrant.
Add the halved Brussels sprouts to the skillet and cook for about 10 minutes until they are tender. Season with cayenne pepper, mustard powder, salt, and freshly ground black pepper to taste.
Pour in the heavy cream and mix well, allowing the Brussels sprouts to soak in the flavors.
Transfer the Brussels sprouts mixture to a baking dish. Sprinkle the crumbled bacon, shredded cheddar, and Gruyere cheese evenly over the top.
Place the dish in the preheated oven and bake for 15-20 minutes, or until the cheese is melted, bubbly, and golden brown.
