In medium saucepan, cook zucchini (or squash), covered, in a small amount of boiling water for 3-5 minutes or till tender. Drain well. Spread evenly in 8 x 1 1/2” round baking dish (or 8"square).
In a bowl combine sour cream, egg yolk, flour, onion salt, and pepper. Stir in cheese. In a bowl, beat egg white till stiff peaks form (tips stand straight). Fold beaten egg white into sour cream mixture. Spoon atop squash.
In a small bowl, combine bread crumbs and melted butter. Sprinkle over egg mixture. Bake in a 350 oven for 20-25 minutes or till golden. Makes 6 servings.