In a small bowl, mix cornstarch and water until smooth. Sprinkle beef with salt and pepper. In a large nonstick skillet or wok coated with cooking spray, heat 2 teaspoons oil over medium-high heat. Add beef; stir-fry 2-3 minutes or until no longer pink. Remove from pan. , In same skillet, stir-fry peppers and apples in remaining oil for 2 minutes. Add 1/2 cup green onions; stir-fry 1-3 minutes longer or until peppers are crisp-tender. Remove from pan., Add brown sugar and vinegar to skillet; bring to a boil, stirring to dissolve sugar. Stir cornstarch mixture and add to pan. Return to a boil; cook and stir 1-2 minutes or until thickened. Return beef and pepper mixture to pan; heat through. If desired, serve with rice. Sprinkle with remaining green onion.