Combine the bacon and onion in a large skillet. Cook until bacon is crisp and onion is tender. Remove oil with spoon, reserving drippings in skillet. Combine flour, salt and pepper; cut liver into serving size pieces and coat. Reserving remaining flour mixture.  Fry liver in reserved remaining bacon drippings; place in 8 cup baking dish. Blend reserved flour mixture with drippings in skillet; add milk. Cook over medium heat, stirring constantly, until the sauce thickens and bubbles. Pour sauce over liver; sprinkle with bacon and onion mixture.  Combine the bread crumbs with margarine; sprinkle evenly over casserole. Bake in moderate oven, 350ºF. for 25 minutes or until sauce is bubbly.