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Beat the cream cheese, mustard and lemon juice together in a bowl, then season with a little salt and pepper and set aside.
Butterfly the turkey breast by cutting into one side of it so you can open it like a book. Cover with cling film and use a meat mallet or rolling pin to gently bash it out to a rectangle about 3cm thick, trimming the edges to neaten it up. You can reserve this trimmings to use in another recipe, such as a turkey curry.
Lay a large sheet of baking parchment on your work surface. Lay the prosciutto over it, overlapping the slices to make a rectangular shape larger than the turkey, then place 8 sage leaves all the way down the centre of the prosciutto in a line, lengthways. Lay the turkey, skinned-side down, on the prosciutto and spread liberally with the cheese mixture. Use the edge of the baking parchment to lift and roll the prosciutto and turkey, tucking and rolling it as you would a roulade, to form a long log. Tie at intervals with string. If chilling, wrap well in cling film and put in the fridge. Can be prepared up to this point, then chilled for up to two days.
To cook, heat oven to 200C/180C fan/gas 6. Remove the cling film and put the turkey roll seam-side down on a baking tray. Brush with a little oil and roast for 1 hr, or until the prosciutto has crisped up and the meat is cooked all the way through (a digital probe should read at least 70C). Scatter over additional sage leaves, if you like, and leave to rest for at least 15 mins before carving.