Preheat the oven to 180ºC, fan 160ºC, gas 4. Put the pork loin steaks between two sheets of cling film and bash with a rolling pin until they're 1cm thick. Remove from the clingfilm.
Lay each slice of ham on to a board, put a pork loin steak on top of each one and spread each of them with a tablespoon of the mushroom paste, then a quarter of the cheese and a sprinkling of thyme leaves (reserve a good pinch of thyme leaves for the leeks). Roll each one up and then secure with a cocktail stick if needed.
Heat half a tablespoon of the oil in a pan and cook the pork parcels for 1 minute on each side, until golden. Place on to a baking tray, transfer to the oven and bake for 15 minutes, until the pork is cooked through.
Meanwhile, put the potatoes in a pan of cold water, cover with a lid and bring to the boil. Simmer for 12-15 minutes until tender, then drain and mash with 1 tablespoon of the olive oil and the milk. Season with freshly ground black pepper.
Heat the remaining oil in a frying pan, add the leeks and remaining thyme leaves and cook for 10 minutes. Stir through the spinach leaves for the last minute, and allow to wilt. Slice each pork loin in half and serve with creamy mash and vegetables.