In a medium bowl, whisk together soy sauce, brown sugar, sesame oil, rice vinegar, gochujang, minced garlic, grated ginger, black pepper, and red pepper flakes until smooth.
In a large bowl, combine ground beef and marinade along with sliced onions. Cover and refrigerate for at least 30 minutes.
Heat a large skillet over medium-high heat. Cook the marinated beef mixture for about 8-10 minutes until browned and fully cooked.
Serve hot over cooked rice or in lettuce wraps, garnished with sliced green onions and sesame seeds.