Place butter and onion in 2-quart casserole. Cover tightly and microwave on high 3 to 4 minutes or until onion is tender. Stir in flour, dry mustard and cayenne pepper. Gradually stir in milk and beer.
Microwave uncovered 6 to 8 minutes, stirring after 4 minutes, until boiling.
Stir in Cheese. (Don't let the soup boil after the Cheese has been added, or it will separate.)
Microwave uncovered 3 to 4 minutes or until Cheese is melted and mixture is hot.
Garnish with shredded carrot or popcorn, if desired.
4 servings (1-1/3 cups each); 455 calories per serving
Microwave time: 12 to 16 minutes
If using a 500-watt microwave, you may need to increase microwave times.
This makes for an unusual wintertime lunch, especially fun when garnished with fresh popcorn.