1Remove fat from beef. Cut beef with grain into 2-inch strips; cut strips across grain into 1/8-inch slices. Mix cornstarch and cold water.
2Heat wok or 12-inch skillet over medium-high heat. Add oil; rotate wok to coat side. Add beef and onion; stir-fry about 3 minutes or until beef is brown. Stir in pineapple, sugar, vinegar, bouillon granules and soy sauce. Heat to boiling. Stir in cornstarch mixture; cook and stir 1 minute.
3Stir in bell pepper and tomatoes; cook and stir 1 minute. Serve with rice.