1. In large saucepan over medium heat, melt butter. Add diced vegetable and saute until soft, about 10 minutes.
2. Add flour and mustard. Cook, stirring constantly, for 1 minute. Slowly stir in stock. Bring to a boil and cook over medium-high heat, until thickened, stirring, about 5 minutes,
3. Reduce heat to medium-low and add cheeses, stirring until melted. Add beer, salt and simmer, uncovered, about 15 minutes. Scorches easily.

Even better the second day.