Trim fat from steak. Cut steak diagonally with the grain into 2-inch-thick slices. Cut slices diagonally across the grain into thin strips. Combine steak, soy sauce, cornstarch, sherry, and sugar in a bowl; stir until well-blended. Cover and marinate in refrigerator 15 minutes.
Heat 1 tablespoon oil in a wok or large nonstick skillet over high heat. Add bell peppers, onion, salt, and crushed red pepper; stir-fry 1 minute. Remove bell pepper mixture from pan; set aside. Add remaining oil, steak mixture, and gingerroot to pan; stir-fry 2 minutes. Return bell pepper mixture to pan, and stir in water; stir-fry 1 minute. Serve with lo mein noodles.