Cook fresh Brussels sprouts as directed above; drain well.
Meanwhile, in 8-inch skillet or medium saucepan cook chopped onion in butter till tender but not brown.
Blend in flour, brown sugar, dry mustard, and 1 teaspoon salt.
Stir in milk.
Cook, stirring constantly, till thickened and bubbly.
Blend in sour cream.
Add cooked Brussels sprouts; stir gently to combine.
Cook till heated through, but do not boil.