Put the parsnips and celeriac into a saucepan and add boiling water. Cook until the vegetables are completely tender, about 15 minutes. Drain, reserving the cooking liquid.
Peel the potatoes and quarter or halve them. Boil these until tender then drain. Put the potatoes back in the saucepan, scrumple a clean tea towel on top of them and cover with a lid.
Set on a very low heat to dry out for 3-4 minutes – this makes better mash (don’t burn the potatoes, though).
Purée the parsnips and celeriac in a food processor, using a little of the cooking water if you need to.
Mash the potatoes with the hot milk then add the vegetable purée, butter, seasoning and nutmeg. Taste to decide whether you would like to add the cream, and for seasoning – mash needs a lot.