Begin by heating a large skillet over medium-high heat. Sear the short ribs for about one minute on each side to create a rich, golden-brown crust. Once seared, transfer the ribs to a crockpot, adding the potatoes, carrots, and sesame seeds.
In a medium-sized bowl, combine the onion, garlic, apple, beef stock, soy sauce, black vinegar, brown sugar, sesame oil, ginger, paprika, chili flakes, and black pepper. Stir until well-mixed, and pour the sauce over the short ribs and vegetables in the crockpot. Set the crockpot to low and cook for 8 hours.
After 8 hours, remove the short ribs and vegetables from the crockpot. Skim off as much fat as possible from the top of the sauce. Add the cornstarch to the remaining liquid and whisk thoroughly until the sauce thickens. You may also want to trim any excess fat from the meat.
Return the short ribs and vegetables to the crockpot and cook for an additional 30 minutes to allow the sauce to reach the desired consistency.
Serve the dish hot, garnished with a sprinkle of sesame seeds and freshly sliced green onions for added flavor and visual appeal.