Trim excess fat from beef short ribs and pat dry. Season generously with salt and pepper.

Heat oil in a skillet over medium-high heat. Brown each side of the ribs for 3-4 minutes until golden.

In a mixing bowl, combine soy sauce, hoisin sauce, mirin, brown sugar, minced garlic, and ginger to create marinade. Add chili flakes or Sriracha if desired.

Place seared ribs in a Dutch oven, pour over marinade, add sliced onions and green onions. Cover and roast at 275-300°F (135-150°C) for 3-4 hours.

Remove ribs and strain braising liquid into a skillet. Simmer until thickened, about 10-15 minutes. Optionally whisk in cornstarch slurry.

For an optional caramelized finish, broil ribs on a baking sheet with reduced glaze for 2-3 minutes.

Serve ribs over steamed rice or noodles, drizzle with glaze, and garnish with green onions, sesame seeds, or cilantro.