In a large bowl, combine the beef, soy sauce, brown sugar, sesame oil, rice vinegar, ginger, garlic, and black pepper. Mix well and let marinate for 30 minutes to 1 hour.
Heat a large skillet or wok over high heat. Add the marinated beef and cook for 5-7 minutes, stirring frequently, until the beef is cooked through and slightly charred.
Remove from heat and stir in the green onions and sesame seeds.
Serve immediately with rice, if desired.
