Preheat oven to 425 degrees F.
Butter casserole dish. In a large pot, melt
3 tablespoons butter and flour over medium heat until golden in color to make a quick roux
. The roux should resemble the color of peanut butter
. Add mushrooms, onion, garlic, garlic powder, cayenne pepper, heavy cream and chicken stock
. Add broccoli
, 1 cup of the cheese
and rice. Season with salt and pepper, to taste. Pour into buttered dish and top with remaining shredded Cheddar
. Bake until cheese is melted and golden, about 20 minutes.