Dredge the ox cheeks in flour and heat the oil in the casserole dish. Fry the cheeks until golden-brown all over, then remove from the dish using a slotted spoon.
Add the vegetables to the casserole and cook for 2-3 minutes. Pour in the wine and scrape up any browned bits with a wooden spoon.
Add the stock and bring to boil, return the ox cheeks to the dish, bring to a simmer and cook for 2-3 hours, or until the ox cheeks are very tender.
Remove the cheeks from the sauce and boil for 4-5 minutes, or until the sauce has thickened. Strain the sauce, then return the ox cheeks back to the sauce.
Meanwhile, for the relish, blend all the ingredients in a food processor until well combined. Spoon the butter onto a large piece of clingfilm, roll into a long sausage, then chill in the fridge.
Serve the cheeks a slice of relish.