Pepper Steak Stir Fry

• 1 lb. top round steak
  3 tablespoons oil
  1 cup beef stock
  1 medium onion, cut into quarter-inch slices
  1-2 garlic cloves, finely chopped
  1 medium green bell pepper, cut into half-inch wide strips
  1 medium red bell pepper, cut into half-inch wide strips
  8 oz. fresh mushrooms, sliced
  1 tablespoon cornstarch
  2 tablespoons soy sauce
  2 medium tomatoes, cut into thin wedges 6 cups hot cooked rice

Cut beef into thin strips for stir fry. Heat oil in wok over medium-high heat. Cook beef in oil until brown.
Stir in beef stock, onion, and garlic. Heat to boiling; reduce heat to low. Cover and simmer 12 to 15 minutes, adding bell peppers and mushrooms during last 5 minutes of simmering, until beef is tender and peppers are crisp-tender. 

Mix cornstarch and soy sauce; stir into beef mixture. Cook, stirring constantly, until mixture thickens and boils. Boil and stir 1 minute.
Cut tomatoes into wedges and place on beef mixture. Cover and cook over low heat about 3 minutes or just until tomatoes are heated through.
Serve over rice.