Preheat the oven to 200˚C/ 400°F/Gas Mark 6.
Layer the potato slices and onion across the base of a large baking tray. Sprinkle generously with Parmesan cheese and season with sea salt and ground black pepper.
Place the rosemary, sage, fennel seeds, garlic, lemon zest in a pestle and mortar and mix until broken down and well combined.
Using a sharp knife, butterfly the pork loin and sprinkle with the chopped herbs. Roll up the pork and wrap tightly with the prosciutto slices.
Place the wrapped pork on top of the potatoes in the roasting tray and roast in the oven for 35 minutes or until the pork is cooked through and golden and the potatoes are crispy.
Whisk together the oil and vinegar in a bowl and season with sea salt and ground black pepper.
Remove from the oven and slice the pork into 1cm thick medallions. Toss the watercress in the vinaigrette until coated and serve with the pork and potatoes.
