In a large mixing bowl, combine soy sauce, brown sugar, sesame oil, rice vinegar, garlic, ginger, and red pepper flakes.
Add the thinly sliced beef to the marinade, tossing to coat well. Cover and refrigerate for at least 30 minutes, preferably 2 hours.
Heat a large skillet or grill pan over medium-high heat.
Add marinated beef and sliced onions to the pan, cooking for 2-4 minutes per side until the beef is cooked through and slightly caramelized.
Sprinkle with chopped green onions and sesame seeds before serving.
Serve hot over steamed white rice.