Preheat the oven to gas 6, 200ºC, fan 180ºC. Cut a cross on the base of each chestnut and spread them out on a baking sheet.
Roast for 25-30 minutes until the skins come away easily and the chestnuts are tender. Remove the skins and roughly chop the nuts.
Heat the oil in a frying pan and fry the onion for 5 minutes. Add the garlic, mushrooms, thyme and 100g (3½oz) of the chopped chestnuts. Cook on a very low heat for about 20 minutes. Add the sherry and turn up the heat, letting all the liquid bubble off.
Put the mixture in a blender, along with the crème fraîche and single cream, and whizz to form a coarse pâté.
Serve on toast, garnished with the remaining thyme and chopped chestnuts.