I first had celeriac mash in France with lamb shanks, and it goes very well with such a dish. I also serve it with Boeuf Bourgignon, ( see my step by step recipe)..
Celeriac is a root vegetable, very ugly, that tastes like celery.
For my celeriac mash I used half a celeriac and 4 medium sized potatoes.
Peel half a large celeriac, and peel the potatoes. Cut both up into chunks
Pop the celeriac into a saucepan of acidulated water ( I squeeze in half a lemon) as it goes brown very quickly.
Pop the potatoes into a separate pan. Bring to boil and cook for twenty minutes until cooked.
Bring the saucepan with the celeriac in to the boil, and boil for about 15 minutes until tender.
Drain the potatoes. Put the dry saucepan back onto the heat and dry out the potatoes by shaking the pan until the potatoes are fluffy.
Using a potato ricer mash the spuds. The ricer is the best tool, as it only squeezes each potato molecule once, so the potatoes never go slimy as can happen if you mash them in a whizzer, and it’s easier to to than with a fork than or a potato masher.
Mash the celeriac with the ricer ( the strong armed person in you household will have to do this as they are hard to mash.
Mix the two mashes together and and butter and hot milk to taste. Whip up. Add salt and pepper.