Trim fat from meat, cut beef across the grain in strips.
In a resealable zip bag, combine soy sauce, garlic and ginger. Add beef, toss and set aside while preparing vegetables.
Heat oil in a skillet. Add beef and stir fry until brown. Cover and simmer meat for 30 to 40 minutes over low heat.
Turn up heat and add vegetables. Toss until the vegetables are crisp tender, about 10 minutes. Mix cornstarch with 1 cup of water. Add to the pan. Stir and cook until thickened. Add tomatoes and heat through.