Cheesy Mushroom and Broccoli Casserole

Name: Cheesy Mushroom and Broccoli Casserole
Ingredients: 3 tablespoons butter, plus extra for casserole dish; 2 tablespoons all-purpose flour; 1/2 pound shiitake or baby bella mushrooms, sliced; 1/4 cup onions, chopped; 2 cloves garlic, finely chopped; 1/2 teaspoon garlic powder; 1/2 teaspoon cayenne pepper; 1 1/2 cups heavy cream; 1/2 cup chicken stock; 1 (10-ounce) box frozen chopped broccoli, thawed and drained; 2 cups shredded Cheddar-Monterey blend; 3 cups cooked rice; Salt and freshly ground black pepper