In a large skillet, heat the vegetable oil over medium heat. Add the chopped onion and cook until softened, about 3-4 minutes.
Stir in the minced garlic and cook for an additional 1 minute until fragrant.
Add the ground beef to the skillet, breaking it apart with a spatula. Cook until browned and cooked through, about 5-7 minutes. Drain any excess fat.
In a small bowl, whisk together the soy sauce, brown sugar, sesame oil, rice vinegar, ground ginger, black pepper, and red pepper flakes (if using).
Pour this sauce over the cooked beef mixture, stirring to combine. Let it simmer for about 5 minutes, allowing the flavors to meld together.
Toast the hamburger buns if desired. Spoon the Korean beef mixture onto the bottom half of each bun.
Top with sliced green onions and a sprinkle of sesame seeds. Place the top half of the bun on and serve immediately.