Pour 2 tablespoons soy sauce and sugar over sliced steak. Marinate at least 30 minutes or up to 4 hours. While the beef is marinating, prep the remaining ingredients.
In a small bowl or 2 cup measure, combine the cornstarch, beef broth, the remaining 2 tablespoons soy sauce. Whisk until blended. Set aside.
Heat 1 tablespoon vegetable oil in large skillet over medium-high. Add half the beef and cook until browned around the edges. Turn the beef slices over and continue cooking for about 2 minutes more. Remove to a plate and keep warm. Repeat with another tablespoon of oil and more sliced beef until all meat is browned.
Pour the last tablespoon of oil into the pan. Add the bell pepper, onions and black pepper. Sauté until the vegetables are crisp tender and slightly charred, about 5 to 8 minutes.
Reduce the heat to medium. Add the in the garlic, ginger and crushed red pepper flakes, if using. Sauté, stirring constantly until the garlic is fragrant, about 1 minute.
Return the cooked beef to the pan, along with any accumulated juices, and the beef broth mixture. Cook until the sauce thickens, about 3 to 5 minutes. Gently fold in the tomatoes and stir to coat. Remove from the heat and serve immediately.
Garnish with additional black pepper and parsley.
Serve over hot cooked rice. Pass additional soy sauce for serving.