When onions and mushrooms are ready, sprinkle flour over vegetables. Cook 1 to 2 minutes or until flour browns and begins to smell nutty. Return beef to pan, and stir in stock mixture. Return to simmering; cook 3 to 4 minutes or until thickened. Place about 1/3 cup mixture over bottom of each bun, top with 1 tablespoon cheese, then top bun.