Sweet and Sour Beef

1 lb beef flank steak
3 Tbsp vegetable oil or chicken or beef broth
1 medium onion, cut into wedges
1 can (8 oz) pineapple chunks, undrained
¼ C sugar
¼ C white vinegar
1 Tbsp instant chicken bullion
1 Tbsp soy sauce
1 Tbsp cornstarch
1 Tbsp cold water
1 medium green pepper, cut into
2 medium tomatoes, cut into eighths

Cut beef with grain into 2-inch strips. Cut strips across grain into 1/8-inch slices. Heat oil in 12-inch skillet or wok over medium-high heat until hot. Add beef and onion; cook and stir until beef is brown, about 3 minutes. Stir in pineapple, sugar, vinegar, bullion (dry), and soy sauce. Heat to boiling. Mix cornstarch and cold water; stir into beef mixture. Cook and stir one minute. Stir in pepper and tomatoes; cook and stir one minute. Serve with hot cooked rice.