Partially freeze beef; cut on the bias into thin slices. For marinade, stir together soy sauce, water, garlic, and pepper. Add beef, stirring to coat well. Cover and marinated at room temperature for 30 minutes or in the refrigerator for 2 hours, stirring occasionally. Drain beef, reserving marinade. Add enough water to reserved marinade to make 1/3 cup. Stir in cornstarch. Set cornstarch mixture aside.
Preheat a medium skillet over high heat; add oil. Add pepper pieces, green onion, and mushrooms; stir-fry about 2 minutes or until vegetables are crisp-tender. Remove vegetables from skillet. (Add more oil as necessary during cooking.)
Add beef to the skillet. Stir-fry for 2 to 3 minutes or until done. Push mixture from center of the skillet.
Stir cornstarch mixture; add to center of the skillet. Cook and stir until thickened and bubbly. Cook and stir for 30 seconds more. Return vegetables; stir ingredients together. Stir in tomatoes. Cook and stir for 1 minute more. Serve over hot cooked noodles, if desired. Makes 1 serving.