In a large saucepan on a medium heat, soften the celeriac pieces in the butter for 5 minutes covered with a lid. Add the milk, salt and pepper and cook on a low heat for 10 minutes until the celeriac is completely soft. Remove from the heat, and with a slotted spoon, place all the celeriac in a food blender with a little of the hot milk and blend to a smooth puree. Make sure you do this on a gradual speed and build up to full power to prevent the puree from being thrown up the side of the blender, and never fill your blender over halfway with hot liquid. Once the puree is smooth, add a little squeeze of lemon juice and put to one side until required or store in a sealed container in the fridge.