1 litre stock, chicken, vegetable or made from the bones of your Christmas bird
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Method
step 1
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).
step 2
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.