Bring a kettle to the boil, then pour the
water over the dried porcini just to cover.
Heat half the butter in a saucepan, then
gently sizzle the onion, garlic and thyme for
5 mins until softened and starting to brown.
Drain the porcini, reserving the juice, then
add to the onion with the mixed wild
mushrooms. Leave to cook for 5 mins until
they go limp. Pour over the stock and the
reserved juices, bring to the boil, then
simmer for 20 mins. Stir in crème fraîche,
then simmer for a few mins more. Blitz the
soup with a hand blender or liquidiser, pass
through a fine sieve, then set aside.