1/2 cup butter or margarine
1/2 cup diced celery
1/2 cup diced carrot
1 cup diced yellow onion
1/2 cup all purpose flour
1/2 teaspoon dry mustard
5 cups chicken broth
3 Tablespoons freshly grated Parmesan Cheese
2 cups firmly packed grated cheddar cheese
12 ounces flat beer
1/2 teaspoon salt
freshly ground black pepper
Melt the butter in a large saucepan, over medium heat. Add the celery, carrot, and onion and saute until soft, about 10 minutes. Stir in the flour and the mustard, and cook, stirring constantly, for 1 minute. Slowly stir in the broth. Bring to a boil and cook over medium-high heat, stirring constantly, until thickened, about 5 minutes.
Reduce the heat to low, add the cheeses, and stir until melted. Add the beer and the salt and simmer, uncovered, about 15 minutes. Stir occasionally and do not allow the soup to boil. Season with pepper.
The soup may be served immediately after cooking or cooled and refrigerated until the next day. Reheat the soup carefully, over medium-low heat and stir often to prevent scorching.