2) Meanwhile, put the diced celeriac in a saucepan with enough water to cover, bring to a boil, loosely cover and reduce the heat to a simmer. Cook until tender, about 30 minutes. It should be the same consistency as the cooked potatoes.
3) Drain the potatoes. Put them through a ricer and add half of the butter and half of the cream.
4) Drain the celeriac and put it in a blender with the remaining butter and cream. Puree* until smooth, using care, as the puree will be very hot.
5) Add the celeriac puree to the potatoes and mix well. Lightly season with salt and pepper.
*Cook's Note: When blending hot liquids, remove the liquid from the heat and allow it to cool for at least 5 minutes. Transfer the liquid to a blender or food processor and fill it no more than halfway. If using a blender, release one corner of the lid. This prevents the vacuum effect that creates heat explosions. Place a towel over the top of the machine, pulse a few times then process on high speed until smooth.
A viewer, who may not be a professional cook, provided this recipe. The Food Network Kitchens chefs have not tested this recipe and therefore we cannot make representation as to the results.