1 litre stock, chicken, vegetable or made from the bones of your Christmas bird
Melt the butter in a large, heavy-based pot over a medium-low heat. Add the celeriac, leek, potato, garlic and onion, season generously, and gently sweat the vegetables until they're all starting to soften (this will take about 10 minutes).
Add the stock, bring the soup up to a boil, turn down the heat and simmer for about 20 minutes or until the celeriac is completely tender. Liquidise until smooth, return to the pan and reheat over a medium flame. Just before serving, check the soup for seasoning and serve.