How to Make It
In a large pot, combine the stock with the sauerkraut and its brine. Stir in the mushrooms, prunes, leek, onion, wax peppers, garlic, serrano chiles and a generous pinch each of salt and pepper. Add the duck legs and sausage and bring to a boil. Simmer over moderately low heat until the duck is tender, about 1 hour.
Remove the soup from the heat and stir in the tomatoes. Cover and let stand until the tomatoes are softened, about 30 minutes. Bring the soup back to a simmer and season with salt and pepper. Serve over mashed potatoes.
Gyulai, a Hungarian smoked sausage, is available at Hungarian butchers and amazon.com. Spanish chorizo will also work in this recipe.