In a medium bowl, combine together soy sauce, sugar, hoisin, ginger, sesame oil, beef broth, garlic and cornstarch; whisk until well incorporated. In a large Ziploc, place sliced steak and pour the sauce over. Transfer to the refrigerator for at least one hour.
Using a slotted spoon, transfer beef in a bowl; set aside both beef and marinade. In a wok over medium heat, add oil and when hot, add beef and cook until the outsides are brown but very pink in the middle, about 2 minutes. Remove from heat and transfer to a clean bowl; set aside.
Return wok on the heat and add sesame oil. When hot, add onion and celery; cook for 3 minutes. Add peppers and cook for 3 minutes. Add broccoli and sauté for 2 minutes. Add bean sprouts and sauté for 2 minutes. Return beef to wok and pour the reserved marinade. Add black pepper and red pepper flakes, cover and steam for 2-3 minutes or until beef is warm through. Serve over a bed of rice and sprinkle with sesame seeds.