Peel the celeriac as you would a swede by removing the outer skin with a heavy knife.

Dice the peeled celeriac into medium size pieces.

Either finely chop the rosemary or leave it as a sprig. Your preference.

Put the diced celeriac, butter and rosemary in a pot and add 100ml of water.

Put a lid on the pot and cook gently until the pieces start to break up.

Add a little more water if necessary to stop it sticking. Your aim is to evaporate the water as the celeriac cooks so that you end up with a nice creamy mash.

When the celeriac is breaking up, finish it off with a masher.

Season to taste