Halve the parsnips lengthwise. (If the core is darker than the surrounding flesh, it could be a bit woody and should be removed.) Roughly dice and put into a large pan. Cover with salted water and bring to the boil, then simmer for about 15 minutes or until the parsnips are tender. Drain, reserving some of the cooking water. Allow the parsnips to cool slightly.
Transfer the parsnips to a blender or food processor, with the double cream and enough of the cooking liquid to loosen the mix. Blitz until the parsnips are smooth.
Just before serving, return the mash to the pan and place over a low heat. Season with the salt and the nutmeg, then add the butter and warm gently, stirring until heated through. Scatter with the spiced breadcrumbs, below, if you like.