Wash and trim sprouts.
Cut any large sprouts in half.
In saucepan, cook sprouts, covered, in a small amount of boiling salted water for 10 to 15 minutes or till just tender; drain.
Meanwhile in medium saucepan, cook onion in butter till tender but not brown.
Stir in flour, brown sugar, salt, and mustard till blended.
Stir in milk.
Cook and stir till mixture thickens and bubbles; blend in sour cream.
Add sprouts; stir gently to combine.
Cook till heated through.
Top with parsley.