Melt butter in a large pot over medium-low heat. Add celeriac, leek, potato, garlic, and onion, seasoning generously.
Sweat the vegetables for about 10 minutes until soft.
Add stock, bring to a boil, then simmer for 20 minutes or until the celeriac is tender. Blend until smooth, return to the pan to reheat. Check seasoning, then serve with a drizzle of pesto.
This recipe originally appeared on River Cottage Christmas Fayre.
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