Trim fat from meat.
Pound meat with meat mallet; sprinkle with salt and pepper.
Place meat in 11 3/4 x7 1/2 x 1 3/4-inch baking dish.
Top with onion and mushrooms; pour sauce over all.
Cook, covered, at 350° for 1 1/2 hours.
Uncover; cook 15 minutes longer.