Melt butter; blend in flour.
Add milk and simmer, stirring constantly, until thickened.
Add onion and fish.
Cut tops off rolls; remove inside of roll without breaking crust.
Brush the inside of each roll with melted butter and fill with fish mixture.
Cover open end of each roll with some of the bread removed from inside of roll and dot with butter.
Bake in a 375° oven until crumbs are browned