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In an appropriately sized pot, slowly fry the onion in the olive oil for about 5 minutes until translucent and soft but not coloured at all. Get your bunch of thyme, tie it up with a little string and add to the pot with the celeriac, potatoes and stock. Bring to the boil and simmer for 40 minutes until the vegetables are tender. Add the cream, bring back to the boil, then remove the thyme and purée the mixture in a liquidizer or food processor. Season carefully to taste, adding the truffle oil tablespoon by tablespoon – the oil can vary in strength depending on the brand. Divide between your serving bowls. Feel free to improvise by adding croûtons, a little extra cream or, if you're really lucky, some real black or white truffles shaved over the top.
Try this: If you want to give an edge to this comforting soup, try dressing some chopped parsley and celery leaves with a little olive oil and lemon juice. Sprinkle over the soup at the last minute before serving.