Place squash in a large saucepan or Dutch oven; add 1 in. of water. Bring to a boil. Reduce heat; cover and simmer for 5-6 minutes or until crisp-tender. Drain and set aside.
In a large nonstick skillet, saute onions in 1 tablespoon butter until tender. Add garlic; saute 1 minute longer. Add squash; stir gently to combine. Transfer to a 13-in. x 9-in. baking dish coated with cooking spray.
In a large bowl, combine the flour, eggs, egg whites, milk, cheese, salt and pepper. Pour over squash mixture. Bake, uncovered, at 350° for 40 minutes.
Melt remaining butter; toss with cracker crumbs. Sprinkle over casserole; bake 5 minutes longer or until crumbs are golden brown and a knife inserted near the center comes out clean. Let stand for 10 minutes before serving. Yield: 15 servings.