Heat the oil in a pan, add the onion, sauté for approx. 5 mins. until translucent. Add the parsnips and celeriac, cook for a further 5 mins. Pour in the stock, bring to the boil, reduce the heat, cover and simmer for approx. 20 mins. Puree until smooth, return to the pan, bring to the boil, pour in the cream, season.
In a blending cup, puree the oil with all the other ingredients up to and including the parsley. Serve the herb oil and crisps with the soup.