1. In 2-quart casserole, combine onion and butter. Microwave at High for 21/2 to 31/2 minutes, or until onion is tender, stirring once.
2. Blend in flour, salt and pepper. Stir in mushrooms, broth, and bouquet sauce. Microwave at High for 3 to 4 minutes, or until mixture begins to thicken, stirring 1 to 2 times.
3. Stir in beef. Cover. Microwave at Power Leve 7 for 7 to 9 minutes, or until beef is no longer pink, stirring every 2 minutes. Blend in sour cream. Microwave at Power Level 7 for 1 minute, or until heated through. Serve with egg noodles.