Stuffing: add the onion, dried panko breadcrumbs, dried cranberries, pistachio nuts, garlic, fennel, salt and pepper to a food processor and blitz to combine. Remove the lid, scrape down the sides and blitz again. The remove and transfer to a bowl.
Pork: place the pork fillet onto a clean and dry cutting board. Next use a sharp knife to remove the grey skin from the pork and discard. Once done cut the pork along its length and open it like a book.
Flatten: once you have opened the pork cover it will 1-2 layers of cling wrap. Next use a meat mallet to flatten it so that it is reasonable even across the width and breath (in thickness).
Stuff: next take the stuffing and lay it flat across the pork keeping a slight border on all 4 sides.
Roll: once you have stuffed the pork start at the longest edge and roll the pork together. Use toothpicks to secure it in place.
Slow cooker: liberally spray the slow cooker insert with cooking oil. Then place the rolled pork tenderloin into the base. Fold the top and bottom in as necessary. Next pour the chicken stock into the bowl, followed by the balsamic vinegar. Once done season the pork with salt and pepper.
Cook: put the lid on the slow cooker and set the time to either 2 hours on high or 4 hours on low. Once the cook time is up remove the lid of the slow cooker and use a meat thermometer to ensure that the pork is cooked through. If it is let the pork rest, if not cook for a further 15 mins (lid on) and then test again. Note 5
To serve: transfer the cooked stuff pork to a large serving platter and surround it with roast potatoes and steamed green beans. Provide gravy and black pepper.