Creamy Reuben Chowder with Rye Croutons

In a large enameled cast-iron casserole, melt the butter. Add the onion, cover and cook over moderate heat, stirring, until softened, about 5 minutes. Add the ham and sausage and cook uncovered, stirring, until lightly browned, about 5 minutes. Stir in the flour. Add the broth and sauerkraut and bring to a boil. Simmer over low heat for 15 minutes. Stir in the crème fraîche and chives. Serve in deep bowls with the rye croutons and horseradish.