Whisk 1 tablespoon cornstarch, 3 tablespoons soy sauce, 1 teaspoon sugar and 1 tablespoon peanut oil in a large bowl. Add the steak and toss to coat; refrigerate until ready to cook.
Whisk the remaining 1 tablespoon each cornstarch and soy sauce, the oyster sauce and chicken broth in a small bowl; set aside.
Heat 1 tablespoon peanut oil in a large skillet over high heat. Add the ginger, broccoli and the remaining 1 teaspoon sugar and stir-fry 3 to 4 minutes; transfer to a plate. Heat 1 more tablespoon peanut oil in the skillet, add the onion and stir-fry 2 to 3 minutes. Add the tomatoes and cook, turning gently, 2 minutes. Transfer the onion and tomatoes to the plate with the broccoli.
Reduce the heat to medium high; add the remaining 2 tablespoons peanut oil to the skillet. Add the garlic and steak and stir-fry 1 minute. Whisk the sauce mixture, then add to the skillet and simmer 1 minute. Return the vegetables to the skillet; cook until the meat is cooked through, 3 to 4 minutes. Serve with rice, if desired.