Add diced onion, gochujang, dark soy sauce, rice vinegar, brown sugar, garlic-ginger paste, tomato paste, and beef stock to the slow cooker insert. Stir everything together really well to create the flavor base.

Lay the ox cheek on top of the mixture, then gently turn or spoon the mixture over the beef so it's completely coated.

Cover with the lid and set the slow cooker to low for 8 hours. The long cook transforms the beef into tender shreds and infuses all the umami-rich flavors.

After 8 hours, carefully remove the beef and shred it with forks. Return the shredded beef to the slow cooker, then add udon noodles and chopped coriander. Stir and cook on high for about 25 minutes more, until noodles are hot and have soaked up the sauce.

Taste and season with salt and freshly cracked black pepper. Garnish with black sesame seeds before serving.