In a large bowl, combine soy sauce, brown sugar, sesame oil, minced garlic, minced ginger, rice vinegar, gochujang, and black pepper. Whisk until the sugar is dissolved.
Add the thinly 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. Add the marinated beef in a single layer, cooking in batches if necessary to avoid overcrowding. Cook for about 3-4 minutes on each side until browned and cooked through.
Remove from heat and sprinkle with chopped green onions and sesame seeds.
Serve the bulgogi over cooked rice and enjoy!