Mushroom and Potato Soup

Soak the porcini mushrooms in 1 1/2 cups of warm water for at least 30 minutes. Drain in a sieve lined with cheesecloth or two layers of paper towels, reserving the liquid for another dish. Squeeze the porcini, letting any extra moisture fall into the water in which they soaked. Wash them in several changes of cold water and chop them roughly.

Melt the butter in a 3-quart heavy saucepan and when it foams, sift the flour directly into the butter, stirring it gently until it has thickened. Add dried and fresh mushrooms, onion, garlic, and marjoram and cook until all the moisture from the mushrooms has been absorbed and the mixture is dry. Cover and continue simmering over very low heat for one hour, bathing the contents with a little hot water every once in a while to prevent them from sticking.

At the end of that time, add salt and freshly grated pepper along with as much water as is necessary to make a soup, about 3 to 3 1/2 cups. Add the potatoes to the pot, bring to a boil, and cook, bubbling lightly, over medium heat until they are tender when pierced with a fork, about 30 minutes. Just before serving, whisk in the sour cream, grated Parmigiano-Reggiano cheese, and parsley.