* Sirloin Steak is to be boneless and sliced into thin strips. And micro- wave on high (100%)) until meat is no longer pink, 4 1/2 to 5 Minutes. Remove meat from casserole; reserve meat and juices. Add mushrooms and onion to meat juices. Cover and microwave until mushrooms are tender, 5 to 6 minutes. Shake water and flour in tightly covered jar; stir into mushrooms. Sprinkle with dry mustard, salt and pepper. Microwave uncovered to boiling, 30 seconds. Boil until thickened, 1 minute. Stir in reserved meat, then sour cream. Microwave uncovered until hot, about 1 minute. Serve over hot cooked rice or hot cooked noodles.