Trim fat from meat. With a fork, pierce meat on both sides. Place meat in an Oven Cooking Bag. Sprinkle onion soup over meat To seal the bag, tie loosely with a strip of plastic or string, leaving a small space for steam to escape. Place bag in a 2 quart baking dish. Microwave at high (100%) for 5 minutes. Microwave at medium (50%) for 50 minutes. Turn roast over, add mushrooms. Microwave at medium (50%) for 25 minutes. Let stand 15 minutes.