Preheat your oven to full whack, (450 to 500 degrees F).
Slash the fat side of the lamb all over with a sharp knife. Lay half the sprigs of rosemary and half the garlic cloves on the bottom of a high-sided roasting pan, rub the lamb all over with olive oil and season with salt and pepper. Place it in the pan on top of the rosemary and garlic, and put the rest of the rosemary and garlic on top of the lamb. Tightly cover the tray with aluminum foil and place in the oven. Turn the oven down immediately to 325 degrees F and cook for 4 hours - it's done if you can pull the meat apart easily with 2 forks.
When the lamb is nearly cooked, put thepotatoes, carrots and rutabaga into a large pot of boiling salted water and boil hard for 20 minutes or so until you can slide a knife into the rutabaga easily. 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 foil and keep warm over a pan of simmering water.
Remove the lamb from the oven and place it on a chopping board. Cover it with foil, then a kitchen towel, and leave it to rest. Put a large pan of salted water on to boil for the greens. Pour away most of the fat from the roasting pan, discarding any bits of rosemary. Put the pan on the stovetop over medium heat and mix in the flour. Add the stock, stirring and scraping all the sticky goodness off the bottom of the pan. You won't need gallons of gravy, just a couple of flavorsome spoonfuls for each serving. Add the capers, turn the heat down and simmer for a few minutes.
Finely chop the mint and add it to the sauce with the red wine vinegar at the last minute then pour into a pitcher. Add the greens and stalks to the pan of fast-boiling salted water and cook for 4 to 5 minutes to just soften them. Drain and toss with a knob of 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!
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