Preheat oven to 350 degrees F. Spray a 9x13-inch baking dish with cooking spray; set aside.

Melt butter in a large skillet over medium-high heat. Add mushrooms and onion and cook for 5 minutes, or until onion is tender.

Sprinkle flour on top and cook for an additional 2 minutes. Whisk in milk, season with garlic powder, onion powder, salt and pepper. Cook, stirring continuously, for 5 minutes or until thickened.

In a large mixing bowl, combine mushroom sauce, cooked rice, diced chicken, broccoli and 3⁄4 cup shredded cheddar cheese.

Spread mixture in prepared pan and sprinkle remaining 3⁄4 cup cheese on top. Bake for 30-35 minutes, or until heated through and cheese is melted.