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Lay the lamb, skin-side down, on a board – if the joint you bought is tied up, then snip the strings and unroll it. Cut several pieces of string long enough to tie around the lamb, then slide them under the joint so they’re regularly spaced out. Mix the stuffing ingredients together, and season well. Spread the stuffing out along the middle of the lamb, then fold the meat over to form a roll. Tie the string so the roll stays together, but don’t make it too tight or it will cut into the meat. Tie some string lengthways too, if necessary.
Heat oven to 170C/150C fan/gas 3. Season the lamb evenly all over. Heat a little oil in a large frying pan, then brown the lamb all over. Start with the side where the seam meets, as this will stop the lamb breaking up when you carve it.
Tip the veg, garlic and rosemary into a roasting tin and put the lamb on top, seam-side down. Cook the lamb until the core temperature reaches 60C on a meat thermometer – this should take around 1 hr for a 2kg leg. As you rest the lamb, the temperature will continue to rise so don’t overcook it.
Lift the lamb out of the tin and, keeping it warm, rest for up to an hour. Put the roasting tin on the hob. Add the stock, bring to a simmer, then tip the lot into a saucepan, scraping up any bits. Add 500ml water and then simmer again until reduced by half. Pour through a fine sieve and serve with the lamb.