Arrange chicken in 12 x 8-inch microwave-safe baking dish, placing thicker portions towardedges of dish. Top with mushrooms and onion. Pour spaghetti sauce over chicken.
Cover with waxed paper; microwave on HIGH 22 minutes or until chicken is nearly done, rearranging chicken parts once during cooking. Let stand, covered, 5 minutes or until chicken is no longer pink in center. Makes 6 servings