In a shallow bowl, combine 3 tablespoons soy sauce and pepper; add beef. Turn to coat; cover and refrigerate 30-60 minutes. In a small bowl, combine the cornstarch, broth and remaining soy sauce until smooth; set aside., Drain beef, discarding marinade. In a large nonstick skillet or wok, stir-fry the garlic and ginger in 2 teaspoons oil for 1 minute. Add the beef; stir-fry 4-6 minutes or until no longer pink. Remove beef and keep warm. , Stir-fry the peppers in remaining oil for 1 minute. Add mushrooms; stir-fry 2 minutes longer or until peppers are crisp-tender. Stir broth mixture and add to vegetable mixture. Bring to a boil; cook and stir for 2 minutes or until thickened. Return beef to pan; add tomatoes and onions. Cook for 2 minutes or until heated through. Serve over rice if desired.