Fry one-half tablespoon finely chopped onion, three tablespoons butter, and one-third cup chopped mushroom caps five minutes. Add two tablespoons flour, and two-thirds cup cream. Cook until mixture thickens, then add one cup finnan haddie (soaked in lukewarm water to cover forty-five minutes, then separated into flakes), two tablespoons grated cheese, and yolks two eggs slightly beaten. Season with salt and cayenne and pile on circular pieces of toasted bread. Sprinkle with grated cheese, then with buttered, soft bread crumbs, and bake until crumbs are browned. Serve at once.