Directions

1. For the marinade, in a small bowl whisk together 1 tablespoon of the vinegar and 1 tablespoon of the soy sauce. Season steak slices with pepper to taste, and place in a large resealable plastic bag. Pour marinade over steak; seal bag, and turn to coat. Refrigerate at least 2 hours.
2. In a small bowl whisk together remaining 2 tablespoons vinegar and 1 tablespoon soy sauce plus brown sugar, garlic, ginger and red pepper flakes. Set aside.
3. Remove steak from marinade; discard marinade. Heat a large nonstick skillet over medium-high heat. Add 1 teaspoon of the oil; swirl to coat the pan. Working in batches if needed, add steak; cook and stir 2 minutes or until no longer pink. Transfer to a bowl.
4. Heat remaining 1 teaspoon oil in skillet over medium-high heat; stir in vinegar mixture. Return steak to skillet; cook 2 minutes or until heated through. Add onion; cook and stir 3 minutes. Add bok choy; cook and stir 3 minutes. (If the pan is dry, add several tablespoons water.) Remove pan from heat and stir in sesame seeds. Serve over brown rice.