Heat the butter and oil in a large heavy-based frying pan over a medium heat. Seal the pork on all sides, remove and set aside. Remove any excess oil from the pan.
Add the garlic, lemon, sage and milk and bring to the boil.
Season the pork with salt and freshly ground black pepper. Return the pork back to the pan with the fat-side facing downwards. Partially cover the pan with a lid or aluminium foil. Simmer over a low heat for three hours.
Turn the pork over and cook for a further hour, adding a little water if necessary.
To serve, remove the pork from the pan, cut into large chunks and drizzle over the cooking liquid.