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 2 tablespoons butter in skillet over medium-high heat; add onion and garlic, and sauté 5 to 6 minutes or until tender. Remove skillet from heat; stir in squash, 1 cup breadcrumbs, 3/4 cup Parmesan cheese, and next 7 ingredients. Spoon into a lightly greased 13- x 9-inch baking dish.
Melt remaining 2 tablespoons butter. Stir together melted butter, remaining 1 1/2 cups soft breadcrumbs, 1/2 cup Parmesan cheese, and garlic salt. Sprinkle mixture evenly over top of casserole.
Bake at 350° for 35 to 40 minutes or until set.