In a small bowl, mix cornstarch, water, soy sauce, brown sugar and seasonings until smooth.
In a large nonstick skillet coated with cooking spray, heat 1 tablespoon oil over medium-high heat. Add beef; stir-fry until no longer pink. Remove from pan. Stir-fry vegetables in remaining oil until tender.
Stir cornstarch mixture and add to pan. Bring to a boil; cook and stir 1-2 minutes or until sauce is thickened. Return beef to pan; heat through.