1. Melt butter in a 1-quart glass measuring pitcher in microwave oven (about 3 seconds at High).
Add flour and blend to a smooth paste.
Add milk gradually, stirring constantly.
2. Cook uncovered in microwave overn 4 minutes at High, or until thickened; stir vigorously after 2 minutes, then every 3 seconds.
3. Add salt, dry mustard and cheese to sauce; stir well. Cook uncovered in microwave oven 1 minute at High; stir.
Yield: About 1 cup.