Trim excess fat from beef; cut beef with grain into 2-inch strips. Cut strips across grain into 2 inch strips. Cut strips across grain into 1/4 inch slices. Mix cornstarch and 1 tbsp water. Heat oil in 12 inch skillet or work over medium-high heat until hot. Add beef and onion; cook and stir until beef is brown, about 3 mins. Stir in pineapple, 1/2 cup water, the sugar, vinegar, instant bouillon and soy sauce. Heat to boiling. Stir in cornstarch mixture. Cook and stir 1 min. Stir in green pepper and tomatoes; cook and stir 1 min. Serve with hot cooked rice if desired