Preheat the oven to 150°C (300°F/Gas 2). Place the stock, milk, bay leaves, thyme and salt in an ovenproof saucepan and bring to the boil. Add the pork cheeks, cover and bake for 2½ hours or until tender. Allow to cool completely in the liquid. Remove the cheeks from the liquid and discard the liquid. Trim the excess fat from the cheeks, then halve each lengthways to form 2 rectangles. Heat the oil in a frying pan and sear the pork cheeks all over. Serve immediately.