Place butter and onion in 1-quart casserole.
MICROWAVE 1 to 2 MINUTES on HIGH, or until onion is transparent.
Add flour, salt, paprika, mustard and pepper.
Add milk and egg mixture slowly, stirring until smooth.
Stir in corn.
MICROWAVE 8 to 10 MINUTES on '8', or until bubbly.
Let stand 3 to 5 minutes, covered.