Place the dried porcini mushrooms in a small bowl with boiling water and soak for 10-15 minutes. Once softened, strain but reserve the liquid, and dice the mushrooms.

While the porcini mushrooms are soaking, heat oil in a large heavy saucepan and sauté the diced leek and celery over medium heat until soft and golden, about 5-7 minutes.

Add the garlic, thyme, fresh mushrooms and sweet potatoes and stir for a minute until slightly golden as well.

Add the soaked porcini mushrooms together with about half a cup of the reserved mushroom-infused liquid. Follow that with the stock, Tamari sauce, mustard, bay leaf, pepper and lemon zest. Add a little bit more water if the saucepan is overcrowded or if you like your soup on the thinner side.

Stir, cover with a lid and bring to a boil. Reduce the heat to simmering and cook the soup with the lid on for about 15 minutes. After that, remove the lid and cook uncovered for a further 5 minutes.

Finally, transfer to a blender or food processor or use a hand-held immersion stick blender right in the saucepan (off the heat though). Add the lemon juice and nut butter and process until well pureed. Taste and season with extra sea salt if needed (depends on the type of stock you used). You can also add a little extra lemon juice, if you like a bit more zing. If the soup is too thick, you can dilute it with a little warm water.

One last step! Slice the remaining 4-5 mushrooms and panfry them in a little coconut oil or butter until browned and season with a little sea salt. Use those and the fresh thyme sprigs to garnish the soup when served.