1. Preheat the oven to its highest setting. Place the pork in a roasting dish. Dry it with kitchen paper, go over the scored lines of the skin with a sharp knife, then rub the salt into the scores. Mix the sage and lemon zest with half the oil; rub into the flesh. Roast for 25 minutes, then turn down the oven to 180°C, gas mark 4 and roast for 55 minutes (or 25 minutes per 500g). Remove from the oven and rest for 10-15 minutes.
2. Meanwhile, make the purée. Heat the butter in a saucepan, then, when foaming, add the onions and garlic; cover and soften for 5 minutes. Add the celeriac and cook, covered, for another 5 minutes, then add the milk, cover and simmer gently for 20 minutes until the celeriac is soft. Transfer to a blender or food processor, season and blitz until smooth. Return to the pan and stir in the nutmeg and crème fraîche. Reheat gently before serving.
3. Heat the remaining oil in a large frying pan over a medium-high heat. Add the crushed garlic and sliced mushrooms, and cook for 3-4 minutes, stirring regularly. Season and toss with the parsley.
4.Slice the meat and serve with the celeriac purée and mushrooms, plus any juices from the roasting pan.