Melt the butter in a large pot over low heat.
Add the flour and begin stirring
Stir until you have a smooth consistency. This is called a roux and is the foundation for many soups, sauces, etc.
Add the chicken broth, beer, cream, and garlic. Continue to stir. Raise the heat to medium if necessary to get a slight simmer going.
Slowly add the cheeses. Taking time to stir between additions. Make sure the cheese is well blended before adding more.
Add the mustard. A whisk at this point might help you achieve a smooth creamy finish to your soup. Add the cornstarch at this point if you find your soup is too runny.
Serve the soup and garnish as you see fit! ;-)