Cut beef into 3x1-in. strips. In a large skillet, brown beef in oil. Transfer to a 3-qt. slow cooker. Combine the onion, soy sauce, garlic, sugar, salt, ginger and pepper; pour over beef. Cover and cook on low until meat is tender, for 5-6 hours. Add tomatoes and green pepper; cook on low until vegetables are tender, about 1 hour longer., Combine cornstarch and cold water until smooth; gradually stir into slow cooker. Cover and cook on high until thickened, 20-30 minutes. Serve with noodles or rice.