First, prepare the squash and onions by washing and trimming the ends of the yellow squash. Then, cut them into ¼-inch thick rounds. Peel and finely chop two cups of onions.
In a large skillet over medium-high heat, melt two tablespoons of butter. Add in the sliced squash and chopped onions, cooking for approximately 10 minutes until they become tender and fragrant.
Moving on to combining the ingredients for the casserole, start by beating four eggs in a large mixing bowl. Next, add two cups of soft bread crumbs, ten pounds of cooked squash and onions, half a cup of sugar, two sticks of melted butter, one teaspoon of black pepper, and two teaspoons of salt. Mix all the ingredients thoroughly until well combined.
Preheat your oven to 375°F (190°C) and grease a 9×13-inch baking dish with cooking spray. Pour the mixture into the prepared dish, smoothing out the top with a spatula.
To add a delicious breadcrumb topping, mix one cup of soft breadcrumbs with four tablespoons of melted butter in a small bowl. Sprinkle this mixture over the top of the casserole.
Finally, bake the casserole for 30-35 minutes until it turns golden brown and slightly crispy around the edges. Let it cool for a few minutes before serving hot and enjoying its savory flavors.