Mix together the milk and lemon juice then place the pork in a deep casserole pour over the milk mixture. Cover with a lid and leave overnight in a cool place.
Preheat the oven to 180C, 350F, Gas Mark 4. Lift the pork out of the milk mixture (reserving the milk), pat it dry with paper towels and seas with salt and pepper.
Heat the butter in a large frying pan, add the pork and sauté until lightly browned all over. Transfer the pork back into the casserole containing the reserved milk mixture.
Add the onion and garlic to the frying pan and sauté until softened.
Add the contents of the frying pan to the pork dish, scraping up any pan drippings and bake, uncovered, for 2 hours or until the pork is tender.
Transfer the pork to a serving platter and keep warm.
Skim any fat from the sauce and transfer to a saucepan. Bring to the boil then continue to boil until it has reduced to 360ml/12fl.oz. stirring from time to time.
Adjust the seasoning and serve separately with the pork.