Tip the porcini mushrooms into a measuring jug. Pour in 600ml boiling water. Set aside to soak.
Peel and finely chop the onion. Trim and finely slice the celery. Warm a large pan for 1 min, then add ½ tbsp olive oil and the chopped veg. Season with salt and pepper. Cover. Keep the heat low and sweat for 10 mins till the veg are soft and glossy looking.
Meanwhile, finely slice the mushrooms. Add them to the pan with most of the thyme sprigs. Cover and sweat for 8-10 mins till juicy. Pour the porcini and its stock into the pan. Turn up the heat. Bring to the boil. Simmer for 10 mins till the veg are tender.
Take the pan off the heat. Fish out the thyme sprigs. Swirl in the double cream. Ladle the soup into a blender and blitz till smooth, or use a hand-held blender in the pan. Pour the soup back into the pan and gently warm for 1-2 mins. Add a little more salt and pepper if you think it needs it.
Serve the soup in warm bowls, garnished with a few fresh thyme leaves.