I adore celeriac, and here it lends itself perfectly to the deep umami of anchovies, their fierce rasp mellowed by the blanketing cream. I often make this up ahead of time, and leave it in a cool part of the kitchen for a few hours before it goes into the oven. It can also be left, covered, in the fridge for up to two days, making sure you let it get to room temperature before roasting it. Keep some of the celeriac-cooking water, too, in case it needs topping up.