Wash hands with soap and water.
Preheat oven to 425 °F.
Butter casserole dish.
In a large pot, melt 3 tablespoons butter and 2 tablespoons flour over medium heat until golden in color to make a quick roux. The roux should resemble the color of peanut butter.
Wash hands with soap and water after handling raw flour.
Gently rub mushrooms under cold running water. Scrub garlic and onion with a clean vegetable brush under running water.
Add mushrooms, onion, garlic, garlic powder, cayenne pepper, heavy cream and chicken stock.
Add broccoli, 1 cup of cheese and rice.
Season with salt and pepper to taste.
Pour all ingredients into a buttered dish and top with remaining cheese.
Bake until cheese is melted and golden, about 20 minutes.
Serve immediately.