Sausage and Leek Soup

Ingredients

4 medium leeks (white and pale green parts only), halved lengthwise, then chopped
1 large carrot, finely chopped
1 celery rib, finely chopped
1 stick (1/2 cup) unsalted butter
8 cups chicken stock or low-sodium chicken broth (64 fluid ounces)
2 medium boiling potatoes
5 tablespoons all-purpose flour
1 cup thinly sliced smoked kielbasa
2 teaspoons chopped fresh marjoram
White pepper to taste

Preparation

*Wash leeks in a large bowl of cold water, then lift out and drain well in a colander.
*Cook carrot and celery in 1/2 stick butter in a 4-5-quart heavy saucepan over moderate heat, stirring occasionally, until softened, about 3 minutes. Add leeks and cook, stirring, until softened, about 3 minutes. Add stock and bring to a boil, then reduce heat and simmer, partially covered, 15 minutes.
*While stock simmers, peel potatoes and cut into 1/2-inch cubes.
*Melt remaining 1/2 stick butter in a small heavy saucepan over low heat, then add flour and cook roux, whisking, 3 minutes. Remove from heat and add 2 cups simmering stock, whisking vigorously (mixture will be thick), then whisk flour mixture into remaining stock and return to a simmer, whisking.
*Add potatoes, kielbasa, and marjoram and simmer soup, partially covered, until potatoes are tender, 10 to 15 minutes. Season with salt and white pepper.