Heat the oil and butter together in a large pan over a medium heat. Add the onion and cook until soft, stirring occasionally. Add the garlic and cook for another 1 minute.
Add all the mushrooms to the pan along with the thyme leaves and stir everything together. Cook for 4-5 minutes until the mushrooms start to soften.
Pour in the sherry, if using, and turn up the heat. Allow to bubble for 2-3 minutes, then add the chestnuts and the stock and bring to a simmer.
Simmer, uncovered, for 12-15 minutes, stirring occasionally, then purée in a processor or with a hand-held blender until smooth.
Divide the soup between 6 bowls. Top each one with a dollop of crème fraîche and garnish with thyme leaves. Serve with bread.
