Preheat oven to 350°F (175°C).

Cook and stir sliced onion in butter or margarine until tender.

Spread onion mixture in a greased 11x7x1 1/2 inch baking dish.

Top with sliced hard-boiled eggs and shredded Swiss cheese.

In a separate bowl, beat together cream of mushroom soup, milk, prepared mustard, seasoning salt, and pepper using a rotary beater.

Pour the soup mixture over the cheese.

Overlap buttered rye bread slices on top of the casserole.

Bake for 30 to 35 minutes, or until heated through.

Set oven control to broil and/or 550°F (288°C).

Place casserole 5 inches from the heat source and broil for 1 minute, or until the bread is toasted.

For bacon curls, cut 18 bacon slices in half, roll each and secure with a wooden pick.

Broil bacon 4-5 inches from heat for 2 minutes, turn and broil 2 minutes longer, or until crisp.