Melt margarine in a skillet; stir in the flour and gradually add the milk.
Over low heat, cook and stir until thickened.
Combine milk sauce and lima beans in food processor or bowl; whip until pureed.
Add onion, eggs, salt, and pepper; stir to blend.
Transfer to a well-greased baking dish.
Sprinkle with bread crumbs.
Bake at 375°F (175°C) for 20 to 25 minutes.