Heat oven to 350. Cook and stir onion in butter until onion is tender.
Spread mixture in baking dish 11x7x1 1/2 inch. Top with egg slices, sprinkle with cheese. Beat remaining ingredients except bread in bowl with rotary beater. Pour soup mixture over cheese*; overlap bread slices on top of casserole (these are the small rye bread rounds, not full sized slices). Bake 30 to 35 minutes or until heated through.
Set oven control to broil and/or 550. Place casserole 5 inches from source of heat; broil 1 minute, or until bread is toasted. (Party rye can be substituted for caraway rye).
*Do ahead note: To make casserole in advance, prepare casserole as directed except - do not top with bread slices or bake. Refrigerate several hours or overnight. At baking time, place bread slices over top and bake 40 to 45 minutes.
(We always serve this with bacon curls, coffee cake and a fruit cup).
Bacon curls in oven: Set oven for broil. Cut 18 slices bacon in half; roll each and secure with wooden pick. Broil 4 - 5 inches from heat source for 2 minutes. Turn: broil 2 minutes longer or until bacon is crisp.