Discard the tough green part of the leek and the outer leaf. Finely chop the rest and rinse in a sieve under running water.
Cut the top and fronds off the fennel bulb and discard any tough outer leaves. Finely chop.
Peel and finely chop the onion. Rinse the rosemary sprig and finely chop the needles. Peel and crush the garlic.
Sauté the pancetta in a large saucepan then remove and set aside.
Add the oil and butter to the pan. Add the onion, garlic, leek and fennel and a little salt and cook on a low heat for about 5 minutes until soft.
Add the chestnuts, rosemary, bay leaf and two thirds of the pancetta to the pan and stir to combine. Add enough water to cover the vegetables, turn the heat to very low and simmer for 20 minutes.
Remove the bay leaf and blitz the soup to a smooth cream. Add the crème fraiche and a generous grinding of black pepper and serve immediately, scattered with the reserved pancetta.
The Juicy Bits
- The leek, onion and garlic combo adds antimicrobial properties to this dish.