Drain pineapple; reserve syrup. Combine syrup with brown sugar, 3 tablespoons teriyaki sauce, vinegar and ketchup; set aside. Mix ground beef with remaining teriyaki sauce and onion; shape into 20 meatballs. Brown in large skillet; drain. Pour syrup mixture over meatballs; simmer 10 minutes, stirring several times. Dissolve cornstarch in 1/4 cup water; stir into pan with pineapple. Cook, stirring constantly, until sauce thickens and pineapple is heated through.