For the pork, place the water, sugar, salt, rosemary and garlic in a large pan and bring to the boil. Take the pan off the heat and when the liquid is cold add the pork, cover the pan and leave for 10 hours.
Preheat the oven to 150C/300F/Gas 1. Remove the pork from the brine, wash well in cold water then place into a casserole.
Cover the pork with the rapeseed oil, place a lid on the casserole and cook in the oven for four hours. To test that it is cooked, insert a metal skewer into the meat – if it slides in easily, the pork is cooked.
Remove the casserole from the oven and set aside. When the pork is cool, remove it from the oil, form it into a cylinder shape then wrap tightly in cling film. Place in the fridge to chill and firm up.
Meanwhile, cook the potatoes in salted water, drain, add the butter and lightly crush with a fork.
To make the coleslaw, mix all the ingredients together in a bowl and season with salt and freshly ground black pepper.
For the dressing, place all the ingredients except the mustard leaves in a food processor and blend until smooth. Season with salt and freshly ground black pepper.
Cut the pork into 2cm/¾in thick slices and peel off the cling film. Pour a drizzle of rapeseed oil in a non-stick pan and fry the pork until golden-brown on both sides.
To serve, spoon some coleslaw onto the serving plates and place the potatoes beside it. Dress the mustard leaves in a little of the dressing. Place the pork on top of the potatoes, spoon over some dressing and serve the mustard leaves alongside.