Soak the dry mushrooms in water a medium bowl and cover with 3 dl of water. Leave it for at least 45 minutes.
Heat 2 tbsp butter in a saucepan over medium low heat and sauté the onion until softened and translucent.
Mix in the flour and then add the stock. Let is simmer for 5 minutes.
Pour in the milk and the cream fraiche - stir so it's evenly mixed. Add salt and pepper and some drops of lemon juice. Let it cook for another 5 minutes.
Serve with some parsley on top and some fresh warm bread.