Peel the celeriac and cut into small cubes.
Boil a pan of lightly salted water, add the vinegar and celeriac pieces. Cook, under cover, for 8-10 minutes or until the celeriac has softened.
Meanwhile, dry-fry the pine nuts and set aside.
Drain and transfer the celeriac cubes to a blender. Season with salt & pepper, add the nutmeg and add the butter& milk.
Pulse, until you get a smooth purée. Taste, add some lemon juice and more seasoning if needed.
Transfer the mash to a serving dish and scatter the pine nuts on top. Grate some nutmeg over it and serve immediately, or keep it warm until serving.