In a medium bowl, mix together the prunes, shallot, garlic, port, rosemary, salt and pepper. Set aside until required.
Using a sharp filleting knife, trim and remove any silver skin from the pork tenderloin. To butterfly the pork tenderloin: Horizontally slice the pork fillet, slicing through the centre of the meat, but not all the way through. Open the pork fillet like a butterfly. Lay a large piece of plastic wrap on the bench. Place the butterflied pork onto the plastic wrap, cover with another sheet of plastic wrap. Pound the meat until it is approximately 7mm thick, trying to keep the meat an even thickness. Remove the top layer of plastic wrap.
Lay a fresh sheet of plastic wrap on the bench. Vertically lay the prosciutto onto the plastic wrap, slightly overlapping the prosciutto. Flip the pork tenderloin on top of the prosciutto, removing the plastic wrap that was under the tenderloin.
Spread the prune mixture evenly over the pork tenderloin. Starting at the longest edge, roll up the pork tenderloin tightly, using the plastic wrap as you roll. Once rolled, refrigerate for at least two hours to ‘set’ the meat. Alternatively tie the tenderloin with twine to keep it together when cooking.
Prepare the barbecue for indirect cooking over medium heat (190°C to 230°C). If using a Weber ® Q barbecue, set up your barbecue with a convection tray and a trivet.
Roast the pork over indirect medium heat, with the lid closed, until cooked through, about 20 minutes.
If desired, move the pork tenderloin over direct medium heat (180°C to 230°C) for 2 minutes each side, to crisp up the prosciutto.
Remove from the barbecue and leave to rest for 5 minutes before slicing.