Microwave 4 Tbsp. butter in a large microwave-safe bowl at HIGH 1 minute or until melted. Add onion, and cover bowl tightly with plastic wrap, folding back a small edge to allow steam to escape. Microwave at HIGH 2 minutes.
Uncover, add squash, and cover bowl tightly with plastic wrap, folding back a small edge. Microwave at HIGH 5 minutes or until squash is tender. (Do not drain.)
Combine mayonnaise and next 4 ingredients in a large bowl. Stir in squash mixture and cheese.
Spoon mixture into a lightly greased 11- x 7-inch baking dish. Cover dish tightly with plastic wrap, folding back a small edge.
Microwave at HIGH 10 minutes or until casserole is set. Remove and let stand 10 minutes.
Microwave remaining 2 Tbsp. butter at HIGH 1 minute or until melted. Stir together melted butter and breadcrumbs; microwave at HIGH 2 minutes. Sprinkle over casserole.
Note: The onion and squash release excess moisture when cooking; do not drain. The flour in the mayonnaise mixture will help absorb the moisture.