Combine the diced onion, gochujang, dark soy sauce, rice vinegar, brown sugar, garlic ginger paste, tomato paste, and beef stock directly in your slow cooker. Whisk everything together until the sauce is smooth and the brown sugar has dissolved completely. The mixture should be deep reddish-brown and aromatic.

Nestle the ox cheek or your chosen cut of beef into the sauce mixture, making sure it's completely submerged. If any part of the meat is exposed, gently press it down or add a splash more stock. Set your slow cooker to low and cook for 8 hours without lifting the lid. The meat is ready when it's fork-tender and pulls apart easily.

After 8 hours, remove the meat carefully with tongs and transfer to a cutting board. Using two forks, shred the beef into bite-sized pieces (roughly ½-inch chunks), discarding any excess fat or connective tissue. Return the shredded meat to the slow cooker and stir it into the sauce.

Add the udon noodles directly to the slow cooker, breaking them apart gently if stuck together. Stir in the chopped cilantro, then switch your slow cooker to high and cook for 25 minutes until the noodles are heated through and have absorbed some of the sauce. Taste and season with salt and pepper as needed, garnish with black sesame seeds, and serve immediately.