Combine onion, garlic, ginger, broth, soy sauce, sugar, rice vinegar, ketchup, gochujang and sesame oil in a bowl. Stir well to combine.
Salt and pepper the short ribs on each side.
Heat 2 tablespoons of vegetable oil in a saute pan over medium-high heat.
Add short ribs to pan, reduce heat to medium and sear each side of ribs, approximately 3 minutes per side.
Transfer ribs, meat side down to slow cooker and pour mixture from bowl overtop. Set the slow cooker on high and cook for 6-7 hours or until the meat is extremely tender.
To thicken the sauce: Dish out about 4 Tbsp of liquid once the ribs are cooked and combine with cornstarch. Pour mixture back into the slow cooker and stir to incorporate. Allow to cook uncovered for 20 minutes or until sauce starts to thicken.
Turn off the slow cooker and allow it to cool and continue to thicken.
Serve with brown rice, kimchi and braised scallions. Top with sesame seeds and enjoy!