Lay half the rosemary into the bottom of a high-sided roasting tray. Break up the garlic bulb, then scatter in half of the unpeeled cloves.
Slash the fat side of the lamb all over with a sharp knife, then rub with oil, sea salt and black pepper. Place into the tray, then scatter the remaining rosemary and garlic on top.
Tightly cover the tray with tin foil and place in the oven. Immediately turn the temperature down to 170°C/325°F/gas 3 and cook for around 4 hours – it’s done if you can pull the meat apart easily with two forks.
Meanwhile, peel the potatoes and chop into large chunks. Peel the carrots and swede, then chop into small chunks. Prep the greens by separating out the leaves and finely slicing any tougher stalks.
When the lamb is nearly cooked, put your potatoes, carrots and swede into a large pot of boiling salted water and boil hard for 20 minutes, or until tender.
Drain and allow to steam dry, then smash them up in the pan with most of the butter. If you prefer a smooth texture, add some cooking water. Spoon into a bowl, cover with tin foil and keep warm over a pan of simmering water until needed.
Once cooked to perfection, remove the lamb from the oven and place it on a chopping board. Cover with tin foil, then a tea towel, and leave to rest.
Put a large pan of salted water on to boil for your greens.
Pour away most of the fat from the roasting tray, discarding any bits of rosemary stalk. Put the tray on the hob over a medium heat and mix in the flour.
Pour in the stock, stirring and scraping all the sticky goodness from the bottom of the tray. You won’t need gallons of gravy, just a couple of flavoursome spoonfuls each. Drain, finely chop and add the capers, then turn the heat down and simmer for a few minutes.
Pick and finely chop the mint and add it to the sauce with the red wine vinegar at the last minute, then pour into a jug.
Place the greens and stalks into the pan of fast-boiling salted water and cook for 4 to 5 minutes, to just soften. Drain and toss with the remaining butter and a pinch of salt and pepper.
Place everything in the middle of the table, and shred the lamb in front of your guests. Absolutely delish!