Place the pork in an oven proof dish and cover with the mixture, turning to coat. Cover with tin foil and cook at 150C for 5 hours, or until tender and falling apart. Shred using 2 forks and set aside.
Mix gochujang with the mustard. Smear sourdough slices with mustard mixture on both sides. Layer ham, korean pork, two slices of cheese and kimchi onto the slices of bread.
Butter the top of the sandwich and place into a heated pan. Press until the top is golden brown and cheese is melted then flip over, butter the top side and continue to cook on the other side until golden brown and the cheese is melted.