In a large skillet, cook the sliced yellow squash in boiling water for 8 to 10 minutes, or until they are just tender.

Drain the squash well and gently pat them dry between paper towels.

In the same skillet, melt 2 tablespoons of butter over medium-high heat.

Add the finely chopped onion and minced garlic, and sauté them for 5 to 6 minutes until they become tender.

Remove the skillet from the heat and mix in the cooked squash, 1 cup of breadcrumbs, ¾ cup of Parmesan cheese, and the next 7 ingredients.

Spoon this mixture into a lightly greased 13×9-inch baking dish.

Melt the remaining 2 tablespoons of butter.

Combine the melted butter with the remaining 1½ cups of soft breadcrumbs, ½ cup of Parmesan cheese, and the garlic salt.

Sprinkle this breadcrumb mixture evenly over the top of the casserole.

Bake the casserole at 350°F (175°C) for 35 to 40 minutes or until it’s fully set.