Melt the chicken fat in a large saucepan over low heat. Add the flour, blend well and cook until the flour is lightly browned, stirring occasionally with a wooden spoon. Gradually stir in the stock and continue stirring until it boils. Lower the heat and simmer, covered, about 45 minutes, stirring occasionally. Season the soup with salt and pepper and pour it into 5 heated bowls. Top them with croutons, sprinkle with the cheese, and serve. This is a good winter soup, and is a specialty of the Basel area in Switzerland. Although this version is made with chicken fat and stock, it can also be made with ham or bacon fat and ham stock. If you decide to do that, though, take it easy on adding salt.