Preparation
Preheat the oven to 170°C, fan 150°C, gas 3½. Pat the pork dry with kitchen paper, remove the string and slice off the skin and most of the thick fatty layer. Season generously. Heat 1½ tablespoons of oil in a large, lidded casserole or deep ovenproof pan. Once hot, add the pork and sear on all sides until well browned – about 10-15 minutes. Set aside on a plate.
Trim the leeks and slice into rounds. Add to the pan with a further ½ tbsp oil and cook over a low heat for 5 minutes until softened and starting to colour.
Meanwhile, peel and finely slice the garlic. Stir the garlic and flour into the leeks and cook for 1-2 minutes, stirring. Gradually pour over the cider and stock, stirring, then bring to the boil.
Return the pork to the pan, cover with a lid and roast in the oven for 1 hour 30 minutes, turning the pork halfway, until cooked through. Lift the pork onto a plate, loosely cover with foil and set aside to rest. Meanwhile, increase the oven to 190°C, fan 170°C, gas 5. Return the leek and cider sauce to the oven, uncovered, for a further 20 minutes or until thickened.
Meanwhile, slice the mushrooms. Heat the remaining 1 tablespoon oil in a large nonstick frying pan, then season and sauté the mushrooms for about 5 minutes over a medium-high heat until golden brown. Add the kale and cook for a further 2 minutes until bright green and wilted.
Stir the mushrooms, kale, crème fraiche and mustard into the sauce for creaminess and punch, then season well. Carve the pork and divide between plates, spooning over the sauce to serve.