Preheat the oven to 200°C/425°F/gas 7. Put the currants in a bowl, cover with warm water and leave to soak.
Melt the butter and 1 tablespoon of oil in a saucepan over a medium heat. Peel and finely chop the onion and garlic, then add to the pan and sauté for 8 to 10 minutes, or until softened. Meanwhile, peel core and dice the apples, then remove the crusts from the bread and blitz to breadcrumbs.
Add the apple to the pan and allow to soften. Pick the thyme and sage leaves straight into the pan, then drain the currants and add to the pan along with the breadcrumbs. Cook for 3 to 4 minutes, until the bread is golden brown, then remove from the pan and set aside to cool.
Place the pork loin on a wooden board. Score the skin with a sharp knife in a criss-cross pattern. Next, butterfly the loin by laying it, skin-side down, on your board and making a slice down one side of the pork, where the skin slightly comes away from flesh. Continue to make long slices all the way down the pork and slowly roll it out until the end, almost like unrolling a carpet.
Spread the cooled stuffing mixture over the flesh and season well with sea salt and black pepper. Gently roll the loin back into its original form, ending up with the cut side on the bottom and the skin on top.
Tightly tie 3 or 4 pieces of string around the pork to make sure no stuffing leaks out. Season the skin.
Place in a roasting tin, add the cider and 200ml of water and pop in the oven for 15 minutes, before turning down to 180°C/350°F/gas 4 and cooking for 1 to 1¼ hours. Trim and wash the leeks, then chop into large chunks. Shred the cabbage and add to the roasting tin along with the leek chunks for the last 45 minutes.