Prepare vegetables. Cut ends off Celeriac right down to the bulb and peel. Use a good sharp knife, not a peeler, as they tend to be quite knobbly and you want to get below all the stringy bits. Roughly dice. Peel, trim ends and dice Parnsip.
Place in a saucepan with milk and peeled garlic clove. Add water, enough to just cover veggies and place on stove top. Bring to the boil, cover with lid and reduce heat to simmer for 45 mins or until very tender.
Strain off liquid and add butter and puree really well.
Serve as a baby puree or an accompaniment to another meal. Freeze in portions.