Put the pork skin side upon a rack in a large roasting tray and cook in a cool oven-150C/fan 130C/gas 2 for about 3 1/2 hours.
Meanwhile, put the shallots (with skin) in a pan, cover with water and bring to a simmer.
Cook for 1 hour, then drain and cool.
Cut each shallot in half lengthways through the root.
Peel off the skin — still leaving the half root in place.
Heat a little oil and butter in a large pan and fry the cut side of the shallots until they are nicely browned.
Turn and colour the other side.
Serve with the pork.