Preheat the oven to 350 degrees F. Lay the prosciutto in an overlapping row on a work surface. Season the pork with pepper and then set, fat side down, on the prosciutto. Wrap the prosciutto around the pork. Set the pork, fat side up, in a parchment paper-lined roasting pan. Roast 60 to 70 minutes, or until a meat thermometer inserted into the centre of the roast reaches 160 degrees F (70 degrees C). Cool the pork to room temperature, and then chill in the fridge at least 2 hours. Make the mayonnaise by combining all listed ingredients. Thinly slice the pork and arrange on a platter with the pesto mayonnaise alongside.