Peel the parsnip. Cut in longitudinal direction into four quarters. Remove the woody core. Cut the parsnip into pieces of equal size. Cook in milk with a little water. Add some butter, optional. When the parsnip is cooked, scoop with a slotted spoon out of the hot liquid. Put the parsnip in a blender or food processor (or hand blender ) and puree. Add cooking moisture and puree until the desired thickness is reached and a smooth mass has arisen. Season with salt, pepper and lemon juice. Keep warm until use. Or leave to cool and warm on when it's about to be used.