Put a saucepan with water and the celeriac cubes on the heat and bring to the boil. Leave the celeriac to cook for about 30 minutes, or until a fork can be easily inserted into the cubes.
Drain the celeriac and return to the pan. Reduce the heat to low and allow the excess water to evaporate, shaking occasionally.
Add the butter and milk to the pan and mash the celeriac until creamy. Use more butter and/or milk if necessary. Season to taste with salt and pepper.
Notes
You can mash the celeriac either with a stick blender, potato masher or - if you don't have those - with a fork. The result will be different depending on which tool you use.
You can store the celeriac puree in an airtight container for 1-2 days in the fridge. You can freeze it for 3 months.
Mixing the celeriac 50/50 with potatoes is also delicious