Arrange chicken in 12x8" microwave-safe baking dish, placing thicker portions toward edges of dish. Top with mushrooms and onion. Pour spaghetti sauce over chicken. Cover with waxed paper; microwave on High 11 min. Rearrange chicken and microwave on High for 11 more mins on until chicken is nearly done. Let stand, covered, 5 mins or until chicken is no longer pink in the middle.
