Preheat the oven to 170C/150C Fan/Gas 3.
Rub the shoulder of lamb all over with the olive oil. Season with salt and pepper. Arrange the rosemary, garlic and onion in the bottom of a deep sided roasting tin and pour in 200ml/7fl oz water. Put the lamb on top and cover with aluminium foil.
Roast the lamb for 3-4 hours, basting every hour or so with the juices in the roasting tray. After two hours, remove the aluminium foil and continue to cook until the meat is very tender and falling off of the bone. Remove the lamb from the tray and set aside to rest.
Put the Jersey Royals and the mint in a saucepan, cover with water and bring to the boil. Cook for 10-15 minutes and then drain.
To make the wild garlic butter, put the butter in a mixing bowl and add the wild garlic. Season with salt and pepper, then set aside.
For the mint sauce, put the mint, sugar and vinegar in a bowl and mix.
Just before serving add a little of the garlic butter to the warm Jersey Royals.
Shred the lamb on a serving plate and pour over the gravy, serve the Jersey Royals alongside and the mint sauce in a small serving dish.