In small saucepan, combine squash, onion and 1/4 teaspoon salt. Cover with water. Bring to boil. Reduce heat; simmer, uncovered, 2 minutes or until squash is crisp-tender. Drain.
In bowl, beat egg, mayonnaise, sugar, pepper and remaining salt until blended. Stir in cheese and squash mixture. Transfer to greased 2 cup baking dish. Toss cornflakes and butter; sprinkle over top. Bake, uncovered at 350° 25 to 30 minutes or until golden brown and bubbly.
*Reduced-fat or fat-free mayonnaise is not recommended for this recipe.
Onion and Cheddar cheese perk up the rich flavor of summer squash in this comforting casserole. A crispy cornflake-crumb topping adds a little crunch.