Melt butter in pot over medium heat, and sauté onions, celery and carrots until they are tender but not browned, about 5 to 10 minutes. Sprinkle mixture with flour, and cook, stirring, for 3 to 4 minutes more. Slowly whisk in chicken stock, and bring it to a boil, whisking almost constantly. Reduce heat to a simmer, and cook, stirring occasionally, for 45 minutes, or until thickened.

Puree until smooth. Return to pot, bring to a simmer, and stir in cream, cheese and mustard. Avoid boiling the soup, which will break down the cheese: reduce heat to low, and stir until cheese is melted. Season with Worcestershire sauce or hot red pepper sauce, if desired, and salt and pepper. Thin with more stock or cream, if preferred. Serve in bowls garnished with croutons, finely chopped smoked ham or chopped cooked broccoli.