Preheat your oven to 300°F (150°C).
Pat the beef short ribs dry with paper towels and season them with salt and black pepper.
In a large oven-safe skillet or Dutch oven, heat the vegetable oil over medium-high heat. Sear the short ribs on all sides until browned, about 3-4 minutes per side. Remove the ribs and set them aside.
In the same skillet, combine soy sauce, brown sugar, rice vinegar, garlic, ginger, sesame oil, and red pepper flakes. Stir until the sugar is dissolved.
Return the short ribs to the skillet, ensuring they are submerged in the sauce. Cover the skillet with a lid or aluminum foil.
Place the skillet in the preheated oven and roast for 3 hours, or until the meat is tender and easily pulls away from the bone.
Once done, remove the skillet from the oven and let it rest for 10 minutes. Skim off any excess fat from the surface of the sauce.
Serve the short ribs with the sauce drizzled over them, garnished with sliced green onions and sesame seeds.