In your slow cooker, combine diced onion, gochujang, dark soy sauce, rice vinegar, brown sugar, garlic-ginger paste, tomato paste, and beef stock. Stir thoroughly until well incorporated.

Carefully place the ox cheek on top of the sauce mixture in the slow cooker. Ensure the beef is fully submerged in the liquid.

Cover the slow cooker with its lid and set it to cook on low heat for 8 hours.

Once the cooking time is up, remove the beef from the slow cooker and shred it using two forks. Return shredded beef to the slow cooker.

Add the udon noodles directly into the slow cooker and stir well. Cover and switch to high for an additional 25 minutes.

Taste the dish and season with salt and pepper as per preference, and garnish with black sesame seeds before serving.