Thinly slice beef on the diagonal (for easy slicing,place meat in the freezer until slightly frozen);place in a snug-fitting bowl.Combine soy sauce,sherry,garlic,and black pepper;pour over meat,tossing to coat completely.Cover and refrigerate 8 to 10 hours. In a large skillet or wok,heat oil. Add green pepper and onion saute for 2 minutes.Dissolve bouillon cube in boiling water. Add beef and marinate.Bring to boiling point.Reduce heat and simmer,covered for 8 minutes.Blend cornstarch with cold water. Stir into mixture in skillet.Cook and stir until thickened.Cut tomatoes into wedges;add to skillet;stir gently.Cover and simmer, just until tomatoes are hot,about 3 minutes.Serve hot over rice with scallions,if desired.Serves 6 to 8.