Melt the butter in a large saucepan. Add the mushrooms and leek, cover and cook over a medium heat for 5 minutes, stirring occasionally. Add the crème fraîche, mustard, nutmeg and some black pepper and cook, uncovered, for 5 minutes, until almost all the liquid has evaporated.
Transfer the mixture to a blender or food processor, add the cheese, and purée in short bursts until smooth.
Scrape the mixture into a bowl or four individual ramekin dishes and chill for at least 1 hour or overnight before serving with oatcakes and mixed crudités.