Keep the dried mushrooms soaked in hot water for at least 2 hours.
Drain the liquid from the dried mushrooms into a bowl.
Squeeze the dried mushrooms over the bowl to extract the juice.
Discard the dried mushrooms.
Wash the watercress leaves.
Separate the leaves from the stems and put them in two separate bowls.
Chop the button mushrooms in small pieces.
Mix the chopped mushrooms in a saucepan with the vegetable stock.
Add the dried mushroom juice and watercress stems to the broth.
Bring to the boil and remove any froth from the top.
Let it cook over low heat for 10 minutes.
Season to taste with salt, black pepper, and lemon juice.
Serve the soup hot, garnishes with watercress leaves.
