Arrange squash in a steamer basket over boiling water. Cover and steam 12-16 minutes or until fork-tender. Cool 15 minutes.

Preheat oven to 350°F. Melt butter in a small skillet. Add onion; sauté 5 minutes or until tender. Transfer to a bowl. Stir in mayonnaise, next 4 ingredients, and 1 cup of cheese until well blended. Add squash and crashed crackers; stir well. Pour mixture into a lightly greased 8-inch square baking dish. Top wit remaining 1/2 cup of cheese.

Bake at 350°F for 35 minutes or until lightly browned.