In a skillet, cook beef and onion over medium heat until meat is no longer pink; drain. Remove from the heat. Add the broccoli, cheese, rice, green pepper and mushrooms; set aside. In a microwave-safe bowl, whisk together the soup mix and water. Microwave, uncovered, on high for 1-1/2 minutes, whisking occasionally. Pour over beef mixture and stir until combined.
Transfer to a greased 1-qt. baking dish. Bake, uncovered, at 350° for 20-30 minutes or until heated through.