Heat butter and olive oil in a large frying pan. When butter has melted, add onions and garlic and stir for 3-4 minutes, without browning.
Add mushrooms, porcini and soaking liquid, thyme and ajowan and reduce heat to low.
Cook for 25-30 minutes, stirring, until all liquid has evaporated and mushrooms are tender, add salt and pepper.
Blitz mushrooms in a blender or food processor then stir through créme fraiche until well combined. Press into a serving dish and refrigerate for at least one hour before serving.