Start by trimming any excess fat from the beef short ribs, then pat them dry with paper towels. Season both sides generously with salt and pepper to enhance the flavors. Let the seasoned ribs sit at room temperature for about 30 minutes.

Heat a large skillet or Dutch oven over medium-high heat. Once hot, add a splash of oil and carefully place the ribs in the pan. Sear the ribs for about 4-5 minutes on each side until golden-brown.

In a mixing bowl, whisk together the soy sauce, hoisin sauce, mirin, brown sugar, minced garlic, grated ginger, and chili flakes if using. Stir until the brown sugar dissolves completely.

Transfer the seared ribs to a Dutch oven or a roasting pan and pour the marinade over them. Add the sliced onion and green onions. Cover tightly and roast in a preheated oven at 275°F for 3-4 hours.

Once the ribs are done roasting, strain the braising liquid into a skillet and bring to a simmer over medium heat. Cook until the liquid thickens and becomes glossy, about 10-15 minutes. Whisk in a cornstarch slurry if desired.

For added caramelization, preheat the broiler. Place the cooked ribs on a baking sheet and broil for 2-3 minutes, watching closely.

Serve the ribs over steamed rice, noodles, or roasted vegetables. Drizzle with the reduced glaze and garnish with green onions or sesame seeds.