Sauerkraut soup is a German and Eastern European classic. It often is made with meat, but for our vegetarian version, we lean on mushrooms—both fresh cremini and dried porcini—to provide savory depth. Crushing the caraway seeds helps them fully release their flavor into the broth. Either pulse them in an electric spice grinder or bash them with a mortar with pestle. Serve with hearty brown bread smeared with butter.