Preheat oven to 350. Prepare a 8" square pan with cooking spray; set aside. To prepare crust, add one tablespoon margarine, garlic powder, and egg whites to hot rice. Spoon rice mixture into prepared pan. Press firmly across bottom and up side of prepared pan to form a crust. To prepare filling, combine remaining margarine, flour, black pepper, and chicken broth in a saucepan. Cook over medium heat, stirring constantly, or until mixture thickens and bubbles. Add turkey, mushrooms, and onions. Mix well. Pour into crust. Bake for 40 minutes.