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Whizz together the prosciutto, the zest of three and the juice from 11⁄2 lemons, the chopped sage and plenty of seasoning until blended to give a thick paste. Put aside and reserve the rest of the lemons.
Cut each tenderloin lengthways down the centre, but not all the way through. Open out the meat, butterfly style, and flatten slightly. Make about 10 deep slashes in each tenderloin, cutting about three quarters of the way through. Rub the paste over the meat and into the slashes.
Brush the tenderloins with oil. Barbecue the meat, paste side down, for 6-8 minutes. Turn over and cook for a further 6-8 minutes until tender and cooked through.
Transfer the pork to a platter and, while hot, top it with the slices of butter. Leave for a minute or two to melt. Squeeze the reserved lemon halves over the pork (or use fresh ones). Scatter with sage sprigs, then serve cut into thick slices.