Combine bacon and onion. Cook until bacon is crisp and onion is tender. Remove and reserve drippings. Combine flour, salt, and pepper; coat liver. Reserve remaining flour mixture. Fry liver in reserved bacon drippings; cut into serving size pieces. Place in an 8-cup baking dish. Blend reserved flour mixture with drippings in skillet. Add milk and cook, stirring constantly, until sauce thickens. Pour sauce over liver, then sprinkle with bacon and onion. Combine bread crumbs and margarine. Sprinkle over casserole. Bake at 350 degrees for 25 minutes.