Rinse the mushrooms in a sieve and soak in hot broth for 1 hour.
Steam the diced onion in fat until translucent. Then pour the porcini mushrooms with the broth and cook for 20 minutes. Add the cream, bring the soup to the boil again. Scatter the mashed potatoes, stirring constantly with the whisk so that no lumps form. Season the soup with salt and pepper.