Trim bread crusts, cut in half diagonally. Arrange half in 8x8x2" greased baking dish (2 qt. size). Sprinkle with cheese, salmon, and onions. Put on remaining bread triangles.
Combine soup, eggs, and milk, pour over casserole. Cover. Chill 6-24 hours.
Before serving, combine 1/4 cup dry bread crumbs, 2 tablespoons melted margarine, 1/4 teaspoon paprika. Sprinkle over top. Bake in 325° oven till set, about 1 hour.