Mix chicken, onion, eggs, mushrooms, olives and mayonnaise together. Season with salt and pepper. Butter 6 slices of bread. Divide mixture into 6 portions and spread on buttered bread. Cover with remaining slices. Arrange in buttered 9x13-inch baking dish. Mix sour cream and chicken soup together and spoon over sandwiches.
Cover with foil and refrigerate overnight. Remove from refrigerator 1 hour before baking.
Bake, covered, in a 325ºF oven for 45 minutes. Remove foil during last 15 minutes of baking time. Recipe makes 6 to 8 servings.