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 a skillet over medium-high heat; add onion, and saute' 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 casserole. Bake at 350º for 30 to 35 minutes or until set.