Preheat the oven to 350°F (175°C). In a skillet, sauté the onions until translucent. Add the sliced yellow squash and cook until tender, about 5-7 minutes.

In a mixing bowl, combine the cooked squash and onions with the shredded cheddar cheese, sour cream, and beaten eggs. Mix well.

In a separate bowl, mix the bread crumbs, melted butter, and grated parmesan cheese until well combined.

Pour the squash mixture into a greased baking dish. Sprinkle the breadcrumb topping evenly over the casserole. Bake in the preheated oven for 25-30 minutes, or until the top is golden brown.