Cut beef across grain into thin strips, 0 cm thick.Combine soy sauce,garlic and ginger.Add beef;toss.Set aside while preparing vegetables. Heat oil in wok.Add beef,toss over high heat until browned. Taste meat.If it is not tender,cover and simmer for 30 to 40 minutes over low heat. Turn heat up and add vegetables.Toss until vegetables are tender crisp,about 10 minutes. Mix cornstarch with water.Add to pan.Stir and cook until thickened.Add tomatoes.