Add diced onion, gochujang, dark soy sauce, rice vinegar, brown sugar, garlic-ginger paste, tomato paste, and beef stock to slow cooker insert. Stir together really well until gochujang is fully dissolved.
Lay ox cheek on top of sauce mixture. Turn beef or spoon sauce over it so meat is completely coated on all sides.
Cover with lid and set slow cooker to low for 8 hours. Beef will become incredibly tender and sauce will concentrate.
After 8 hours, carefully remove beef and shred into bite-sized pieces with two forks. Return shredded beef to slow cooker and stir into sauce.
Add udon noodles and chopped coriander to slow cooker. Stir gently, ensuring noodles are submerged. Turn heat to high and cook 25 minutes more, stirring occasionally, until noodles are hot and have absorbed sauce.
Taste and season with salt and freshly cracked black pepper as needed. Garnish with black sesame seeds before serving.
Notes
Ox cheek brings gorgeous richness, but chuck roast or short ribs work equally well.
Always add ready-to-use udon at the very end to prevent mushiness.
For thicker sauce, remove lid for last 10 minutes of cooking.
Double the garlic-ginger paste for bigger flavors.
Flavors deepen overnight, making leftovers even better.