Around 30 minutes before cooking, remove the lamb from the fridge and allow to come up to room temperature.
Preheat the oven to 150ºC/300ºF/gas 2. Peel and roughly slice 4 red onions, then place in a deep roasting tray. Halve and add 1 bulb of garlic, then lay the lamb on top.
Drizzle over a little oil, then pick, roughly chop and sprinkle over the leaves from ½ a bunch of fresh rosemary. Finely grate over half of 1 whole nutmeg, add a good pinch of sea salt and black pepper and rub all over the lamb.
Drizzle over 200ml of balsamic vinegar, add a good splash of water to the tray and cover tightly with a double layer of tin foil.
Cook in the oven for 5 hours, removing the foil for the final hour, until the meat is tender and almost falling off the bone and the outside is super-sticky and gnarly – add splashes of water, if needed, and baste with the pan juices occasionally.
Place the lamb onto a serving platter, cover with tin foil and leave to rest while you make the gravy.
Pick and roughly chop the leaves from 1 bunch of fresh mint. Skim away the fat from the top of the roasting tray and place on the hob over a medium heat. Whisk in 1 heaped tablespoon of plain flour, pour in 400ml of boiling water then add the mint and simmer gently for 10 minutes, or until thick and glossy, stirring regularly.
Sieve the minty gravy into a jug and serve alongside the lamb with some potato and celeriac mash and steamed seasonal greens, if you like.