Saute onion in butter for 5 minutes.
Sprinkle with flour and gradually add milk, stirring constantly until mixture reaches boiling point.
Add salt, mustard, Worcestershire sauce, lemon juice, egg, and halibut.
Mix lightly and divide among 6 buttered baking dishes.
Sprinkle with bread crumbs.
Bake in a 350° oven for 20 minutes.