In a medium saucepan over low heat, sweat the celeriac in the oil with a little salt until tender without colouring. Add the milk and simmer for 5 minutes. Remove from the heat and blend until smooth. Pass through a fine chinois (conical sieve) and check the seasoning. Cover with a piece of baking paper to prevent a skin forming and refrigerate until needed.
When ready to serve, gently warm over low heat.