Melt the butter with the oil in a large, heavy-based flameproof casserole or a roasting tin large enough to hold the pork. Season the pork with salt and pepper and add to the pan, fat-side down. Cook over a medium-high heat for 10 minutes until golden brown.
Pour away most of the fat and turn the pork over, skin-side up. Pour in the milk and add the garlic, lemon rind and sage. Bring to the boil, then cover with a lid or foil, leaving a little gap for the steam to escape. Cook on the hob over a very low heat or in a preheated oven, 150°C (300°F), Gas Mark 2, for 1½ hours, basting regularly with the sauce. The pork is ready when the meat feels very tender when pierced with a fork.
Remove the meat and leave to rest for 10 minutes. The sauce, which should be biscuity in colour, will be unattractively lumpy, so vigorously whisk to break up the lumps or process in a food processor or blender until smooth. Reheat if necessary and season with salt and pepper. Separate the loin into 6 chops and serve with the sauce spooned over.