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.