In a large skillet, cook the bacon until crispy. Transfer to a plate, crumble, and set aside.
Add two tablespoons of butter to the skillet and saute the minced shallots and garlic for 30 seconds.
Mix in the brussels sprouts and cook until tender, about 10 mins.
Pour in the heavy cream and season with salt, pepper, cayenne, and mustard powder. Cook until slightly thick, about 3 minutes.
Transfer to a baking dish, and sprinkle the bacon and shredded cheeses evenly on top. Bake for 15 to 20 mins or until the cheeses are melted and slightly brown.