In a bowl, combine soy sauce, brown sugar, sesame oil, vegetable oil, minced garlic, grated ginger, black pepper, and red pepper flakes. Whisk until the sugar is dissolved.
Add the sliced flank steak to the marinade, ensuring all pieces are well coated. Cover and refrigerate for at least 30 minutes, or up to 2 hours for more flavor.
Heat a large skillet or grill pan over medium-high heat. Once hot, add the marinated beef in a single layer (you may need to do this in batches). Cook for 2-3 minutes on each side until browned and cooked through.
Remove the beef from the skillet and let it rest for a few minutes.
Serve the bulgogi topped with chopped green onions and sesame seeds. Enjoy with steamed rice or lettuce wraps.