Step 1Clean the mushrooms and cut them into chunks of roughly even size.
Step 2Melt the butter in a large shallow saucepan over medium heat. Add the leeks, sprinkle with salt and cook, stirring often, until softened, 5 to 8 minutes.
Step 3Add the mushrooms and stir to mix well. Add the thyme, bay leaf and cayenne pepper and mix well. Add the wine and cook, stirring, until the liquid is reduced to a glaze.
Step 4Add the stock and bring to a simmer. Cook, stirring occasionally, until the mushrooms are tender, about 15 minutes (cooking time will depend on variety and age of mushrooms).
Step 5Stir in the creme fraiche and heat through. (Add more if you want more liquid.) Taste and add more salt if needed. Season well with pepper and serve.