1Crumble beef into 2-quart microwavable casserole. Add onion. Cover loosely and microwave on High 5 to 6 minutes, stirring after 3 minutes, until beef is no longer pink; drain.
2Stir in ravioli. Cover tightly and microwave 5 to 7 minutes, stirring after 3 minutes, until hot. Sprinkle with cheese. Cover and let stand until cheese is melted.