1. Combine squash and green onions in a large microwave-safe bowl. Cover tightly with plastic wrap, folding back a small edge to allow steam to escape. Microwave at HIGH 8 minutes or until squash is tender. (Do not drain.)
2. Whisk together mayonnaise and next 3 ingredients in a large bowl; stir in squash mixture and cheese. Spoon mixture into a lightly greased 11- x 7-inch baking dish; cover tightly with plastic wrap, folding back a small edge to allow steam to escape. Microwave at HIGH 10 minutes or until casserole is set; remove from microwave, and let stand 10 minutes.
3. Stir together breadcrumbs and melted butter; microwave 2 minutes. Sprinkle over casserole.