Heat the chilli oil in a large saucepan until a sage leaf sizzles and crisps in 15–20 seconds and fry the remaining leaves in batches until crisp, lifting out with a slotted spoon on to a plate lined with kitchen paper. Set aside.
Add the leek and parsnips to the pan and fry gently for 10 minutes until softened. Add the stock and cloves and bring to the boil. Reduce the heat, cover and cook very gently for 30 minutes until the vegetables are very soft. Stir in the chestnuts and cook for a further 5 minutes.
Blend the soup using an immersion blender or in a food processor. Add the lemon juice and reheat gently, seasoning to taste with salt and pepper.
Ladle into bowls, top with a little crème fraîche and drizzle sparingly with extra chilli oil. Serve scattered with the sage leaves.