Cook squash in boiling water to cover in a large skillet 8-10 minutes or just until tender. Drain well and gently press between paper towels.
Melt 2 tablespoons butter in skillet over medium-high heat; add onion and garlic, and sauté 5-6 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 13x9-inch baking dish.
Melt remaining 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.