In a large pot, heat the olive oil. Add the onion, garlic and thyme, and fry for 5 minutes. Then add the mushrooms and a good teaspoon of salt. Cook on a high heat for about 5 minutes, until they have wilted. Pour in the sherry or white wine and sizzle for a few minutes. Add the chestnuts and stock and simmer for 15 minutes.
Put the ingredients in a blender and whizz until smooth. Pour back into a clean pot and add half the crème fraîche. Stir through and warm. Serve in bowls with a dollop of crème fraîche.