Pour boiling water to cover the dried porcini and allow them to soak for 15 minutes or until soft.
In a large pot, sweat the chopped shallot in the olive oil until soft.
Drain and rough chop the softened porcini mushrooms, tossing any foreign or hard bits but saving the liquid.
Add the sliced mushrooms and porcini to the pot and sautee for 3 minutes.
Add the chicken or vegetable stock, the reserved porcini liquid and the chopped thyme.
Bring to a boil, lower the flame and cook gently for 30 minutes until all mushrooms are soft.
Using a hand blender, puree the soup to desired consistency.
I like to leave a few small chunks of mushroom, but you may prefer your soup smooth.
Taste and adjust for seasoning.
Ladle soup into individual bowls, top with a dollop of creme fraiche or sour cream and a sprig of fresh thyme if desired and serve immediately.