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First, make the stuffing. In a large pan, melt the butter over a gentle heat and add the shallots. Cook slowly for about 10 mins until softening, then add the garlic and stir to combine for 1 min. Turn up the heat and add the pancetta and a little seasoning. Cook until the pancetta is crisp, and the shallots are tender and turning golden. Take off the heat, add the rest of the ingredients and stir to combine (it should not be too wet, as the stuffing should absorb some of the lamb juices when cooked). Set aside to cool.
Heat oven to 200C/180C fan/gas 6. Lay the lamb saddle out on a board, fat-side down. Arrange the stuffing in a line down the middle, bring the sides up and roll into a large sausage shape so the stuffing can only be seen at the ends. Using string, tie the saddle, tucking the rosemary under the string, then rub with salt and a little pepper.
Heat the oil in a large pan over a high heat and place the rolled lamb in the pan to seal the outside, turning every minute or so, until golden – about 6 mins. Remove from the pan and place in a roasting tin. Pour the vinegar over – this will help to crisp the skin – then cook in the oven for 1 hr. Leave to rest for at least 10 mins on a board covered loosely with foil.