In a large bowl/jug, pour the boiling water over the dried shiitake mushrooms and leave for at least 20 minutes.
Remove the mushrooms from the water, squeezing out any excess back into the water. Chop roughly and set aside. Crumble the stock cube into the water.
On a medium heat melt the butter in a pan and add the chopped garlic, leek and celery and sweat until soft (5-8 minutes).
Increase to a high heat, remove the thyme leaves from sprigs and sprinkle into pan along with the chestnut mushrooms and the reconstituted shiitake.
Sprinkle in the umami seasoning and stir before pouring in the stock.
Bring to the boil, and simmer for 15 minutes until the mushrooms are cooked, then return to a low heat.
For the croutons, heat a frying pan with the oil on a medium high heat. Mix the umami seasoning with the oil before lightly frying the croutons until they become golden and crunchy.
Serve the soup with a dash of cream and the croutons.