Place butter in a deep 11/2-quart casserole.
Microwave at HIGH for 45 seconds or until butter melts.
Stir in onions and curry powder; microwave at HIGH for 1 minute or until onion is tender.
Add flour, stirring until smooth.
Gradually add broth and milk, stirring well.
Microwave at HIGH for 4 minutes; stir well with a wire whisk.
Add mushrooms, salt, and pepper.
Microwave at HIGH for 2 to 3 minutes or until slightly thickened.
Combine half-and-half and egg yolk, stirring well.
Gradually add about one-fourth of hot mixture to yolk mixture, stirring constantly; add to remaining hot mixture.
Microwave at HIGH for 1 1/2 minutes, stirring every 30 seconds until smooth and slightly thickened.
Serve immediately.