Preheat the oven to 200ºC/400ºF/gas 6. Lightly grease a large roasting tray.
Peel and coarsely grate the potato, then add to a bowl with the minced pork and beef. Scrunch and mix together, then use wet hands to roll the mixture into balls, slightly larger than a golf ball – you should end up with 21 altogether.
Reserving 3 to one side, place the meatballs into the prepared roasting tray, then set aside.
To make the gravy, peel and finely chop the onions, trim and finely chop the fennel, then add to a large pan over a medium heat with a good splash of oil. Cook for 10 minutes, or until softened.
Add the 3 reserved meatballs, breaking them up in the pan, then cook for a further 10 minutes over a high heat, or until dark golden.
Stir in the flour and cook for 2 minutes, then add the Porter and leave to boil and bubble away.
Pour in the stock and stir in the remaining ingredients, then bring to the boil. Reduce to a simmer and cook for a further 20 minutes, or until thickened and glossy.
Season the meatballs with sea salt and black pepper, then place in the hot oven for 20 to 25 minutes, or until golden and cooked through.
Once cooked, remove the tray from the oven and place over a medium heat on the hob. Pour over the gravy, add the rosemary sprigs and simmer for a few minutes.
Warm the rolls in the cooling oven.
Stir the vinegar into the gravy, grate over the cheese and remove from the heat.
Slice the rolls and open them out like a book, spoon in the cheesy meatballs and gravy (discard the rosemary sprigs), top with watercress, then serve with any leftover gravy for dunking.