Wash and clean the nettles, then prepare a large pasta pot with a steaming basket. Steam the nettles, covered, for five minutes, or until thoroughly wilted. Remove the nettles to a tray to cool, then wring them dry and chop roughly to make sure there’s no long stems.
Meanwhile, chop the carrot onion and celery and sweat in the butter in a soup pot. Add the potatoes and sweat for a few minutes more.
Add the wine, tomatoes and stock and bring the mixture to a simmer, covered, until the potatoes are tender.
Add the nettles and heat through, then puree the soup with a handblender-it should not be perfectly pureed. Adjust the consistency of the soup if needed, adding a little stock if it looks too thick. Adjust the seasoning for salt and pepper until it tastes good to you.
Heat the butter until sizzling in a very small pot or pan. Ladle the soup while it’s quite hot into serving bowls, garnish with a few preserved mushrooms, and spoon the hot butter over the top of each bowl of soup along with the chive flowers if using, and serve.