Cook squash in boiling water to cover in a large skillet 8 to 10 minutes or just until tender. Drain well; gently press between paper towels.
Melt 4 tablespoons butter in skillet over medium-high heat; add onion, and sauté 5 minutes or until tender. Remove skillet from heat; stir in squash, cheese, and next 4 ingredients. Spoon mixture into a lightly greased 11- x 7-inch baking dish.
Melt remaining 1 tablespoon butter. Stir together melted butter and crushed crackers; sprinkle evenly over top of casserole.
Bake at 350° for 30 to 35 minutes or until set.
Note: To lighten, reduce butter to 3 tablespoons, using 2 tablespoons to sauté onion. Substitute 1/2 cup egg substitute for eggs and low-fat versions of mayonnaise, cheese, and crackers. Proceed as directed.