- Slice beef across grain into thin strips. Coat with mixture of cornstarch and soy sauce.
- Heat 2 Tbsp. vegetable oil in wok or large skillet over high heat. Add beef and stir-fry 1 minute; remove.
- Heat remaining 1 Tbsp. oil in same pan. Add onion and bell peppers; stir-fry 2 minutes. Add beef, black bean sauce and 2 Tbsp. water; cook and stir until pieces are evenly coated.