Place dried mushrooms in a small bowl, cover with boiling water and leave for at least 30 minutes or overnight. Drain and finely chop mushrooms, then set aside along with reserved soaking liquid.
Melt ¼ of butter and all of olive oil in saucepan. Add garlic and cook 1 minute, then add onion, celery and thyme and gently cook for 10 -15 minutes. Add diced potato and dried mushrooms and cook until soft (10 - 15 minutes).
Whist onion mixture is cooking, prepare and cook mushrooms: In a separate pan melt 1/3 of remaining butter, add sliced mushrooms in batches and quickly fry off until lightly coloured. Season as you go along. Transfer mushrooms plus any liquid from them to a bowl. Repeat the process until all the mushrooms are cooked.
Add cooked mushroom to onion mixture along with hot vegetable stock; bring to a gentle simmer for about 15 to 20 minutes then remove from heat. Pour half the soup into a blender (or use blender stick), whiz into a purée then pour it back into the pan and return to a simmer. Add cream if using, chopped parsley and season carefully to taste.
Serve soup in warmed bowls, along with some charred sourdough or your favourite brown bread.