Cut steak(s) with the grain into 2-inch strips, then cut across the grain into ¼-inch pieces.
Add beef to a bowl with 1 tablespoon vegetable oil, cornstarch, soy sauce, salt and white pepper. Stir to coat, cover bowl with plastic wrap and refrigerate for 20 minutes.
While steak is marinating, trim tops off bell peppers and remove seeds. Cut peppers into 1-inch pieces. Peel and cut onion into 1-inch pieces. Cut each tomato into 8 pieces. Place all veggies in large bowl or on plate and set aside.
In small bowl, whisk together beef broth, cornstarch, soy sauce and sugar. Set aside.
Heat a wok over high heat. Once hot, add 2 tablespoons oil, the onion, garlic and ginger. Stir fry one minute, then add the beef. Continue stir frying for 3 minutes, tossing occasionally until beef is no longer pink. Remove from wok and set aside.
Add more oil to wok if needed, then stir fry tomatoes for 1 minute. Pour broth and cornstarch mixture, toss to coat tomatoes, and cook until sauce thickens. Add bell peppers and beef (with accumulated juices) to wok and stir fry 1 minute more.
Garnish with toasted sesame seeds and cilantro sprigs (optional). Serve as is, over noodles or with steamed rice.