1. Melt the butter in a heavy soup pot. Add the onion and celery. Cook until just soft. Sprinkle on the flour and cook for three minutes, stirring frequently.
2. Slowly whisk in first the broth, then the beer. Bring to a boil. Reduce the heat. Slowly whisk in the cheese. Add the mustard.
3. Remove from the heat and serve immediately. To reheat, place in a double boiler to prevent scorching.