Smoked Sausage & Sauerkraut Soup

3 quarts chicken stock or low-sodium broth One 25-ounce jar of sauerkraut with brine 1/2 pound shiitake mushrooms, stems discarded and caps sliced 1/2 cup chopped pitted prunes 1 medium leek, white and light green parts only, finely diced 1 onion, finely diced 2 Hungarian wax peppers—halved, seeded and thinly sliced 6 garlic cloves, thinly sliced 2 serrano chiles, seeded and thinly sliced Kosher salt Freshly ground black pepper Four 10-ounce duck legs, skin and excess fat removed 3/4 pound Gyulai sausage, cut crosswise into 1/4-inch slices (see Note) 4 large tomatoes, diced Mashed potatoes, for serving