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Heat the oil in a large pan. Sizzle the pancetta for about 3 mins on each side until crisp, then remove to a plate and set aside. Melt the butter in the same pan, add the onion, bay leaf and thyme stalks, and cook for 10 mins until just starting to turn golden. Add the celeriac and cook for 2 mins more.
Pour over the stock and simmer for 10 mins until the celeriac is soft. Stir in the cream and bring back to the boil. Fish out the bay and thyme stalks, then purée the soup until smooth. Stir through half the thyme leaves and ladle the soup into bowls. Serve topped with the crispy pancetta, the remaining thyme leaves and a drizzle of olive oil.