Preheat oven to 220°C.
Butter casserole dish. In a large pot, melt three 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, double cream and stock. Add broccoli, half of the cheese and all the 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.