Cut steak across grain into thin strips, then into 1-inch squares; coat with 1 Tbsp. stir-fry sauce. Heat 1 Tbsp. oil in hot wok or large skillet over high heat. Add beef and stir-fry about 1 minute; remove. Heat remaining oil in same pan. Add peppers and onions; stir-fry 5 minutes. Stir in beef and 1/4 cup stir-fry sauce; cook and stir just until beef and vegetables are coated with sauce. Serve immediately.