Peel the celeriac with a sharp knife or potato peeler.
Slice into strips and then cut into cubes.
Place in a medium saucepan, add the garlic and cover with cold water.
Bring to the boil, then lower the heat and simmer until tender.
Place in a blender or food processor. Add the butter and a splash of cream and blitz until smooth. If the mash is too thick, add a little more cream or milk.
Stir in the parsley, drizzle with a little olive oil and serve.