Slice steak into thin strips across the grain.
Whisk soy sauce, sugar, cornstarch, and ginger in a bowl. Coat beef.
Heat 1 tbsp oil in a wok. Stir-fry half the beef 3 minutes. Remove. Repeat with remaining beef.
Return all beef to wok. Add onion and stir-fry 2 minutes.
Add bell pepper and cook 2 minutes until crisp-tender.
Add tomatoes, stir, and heat through.
Serve hot over rice or noodles.
