Heat a large skillet over medium-high heat for 3 minutes. Pat ribs dry with paper towels and season with black pepper. Sear ribs in the hot skillet for 2-3 minutes per side until browned and caramelized. Work in batches if the pan is crowded.
3 pounds beef short ribs, 1 teaspoon black pepper
In the slow cooker, whisk together soy sauce, brown sugar, minced garlic, grated ginger, and gochujang if using until the sugar dissolves.
1 cup soy sauce, ½ cup brown sugar, 4 cloves garlic, 1 tablespoon fresh ginger, 1 tablespoon gochujang
Add the seared ribs to the slow cooker and turn them to coat in the sauce. Cover and slow cook on low for 7-8 hours until the meat pulls easily from the bone but is not falling apart.
3 pounds beef short ribs
Carefully transfer the ribs to a plate. Pour the leftover sauce into a skillet and simmer over medium heat for 5-7 minutes until thick and glossy. Skim off any excess fat if needed.
Return the ribs to the thickened sauce and turn to coat well. Drizzle sesame oil over the ribs and garnish with sliced green onions. Optionally, broil ribs for 2-3 minutes about 6 inches from the heat to caramelize edges before serving.
1 teaspoon sesame oil, 2 tablespoons green onions