Preheat oven to 450 degrees F. Slice chicken breasts into cubes. Wash mushrooms. Place two cups rice in the bottom of a 13x9 glass baking dish. Add the two cups water and the cream of mushroom soup. Stir until the soup has mixed in evenly with the water and rice. Add in the frozen broccoli, mushrooms, chicken, minced onions, garlic salt and fresh ground pepper. Stir to evenly coat and mix all. Cover with aluminum foil and place in preheated oven for 45 minutes. Take out and add cheese to top. Bake an additional 15-20 minutes to melt the cheese.