Preheat your oven to 350°F (175°C). Grease a 2-quart baking dish with cooking spray or butter.

In a large pot, bring water to a boil. Add the sliced squash and chopped onion, cooking for about 5 minutes until tender. Drain well and set aside.

In a large mixing bowl, combine the shredded cheddar cheese, crushed crackers, milk, eggs, mayonnaise, salt, black pepper, garlic powder, and onion powder. Mix until well combined.

Fold the drained squash and onion mixture into the cheese mixture until evenly coated.

Pour the mixture into the prepared baking dish. In a small bowl, combine the additional crushed crackers with the melted butter, then sprinkle evenly over the top of the casserole.

Bake in the preheated oven for 30-35 minutes, or until the top is golden brown and the casserole is set.

Remove from the oven and let it cool for about 5 minutes before serving.