In large resealable bag, combine soy sauce, 1/4 cup water, sherry, garlic powder and beef strips. Close tightly and refrigerate 8 hours or overnight.
Combine green peppers, beef and marinade in 2 quart casserole dish; cover.
Microwave on MEDIUM (50%) power until beef is tender, 20-25 minutes, stirring at after the first 10-12 minutes.
Drain, reserving meat juices, and set beef and peppers aside, covered.
Mix cornstarch and cold water in 2-cup glass measure or small bowl. Stir in reserved meat juices and microwave at HIGH (100%) power about 1 minute. Stir and microwave another 1-3 minutes or until the sauce thickens.
Stir sauce and tomatoes into beef mixture; cover and microwave on HIGH (100%) power until tomatoes are heated through, 1-3 minutes.