Preheat your oven to 375°F (190°C). Grease a 9x13 inch baking dish with butter or cooking spray.

In a large pot, bring water to a boil. Add sliced fresh squash and cook for 8-10 minutes until tender. Drain thoroughly.

In a mixing bowl, combine the drained squash, melted butter, sour cream, and shredded cheddar cheese. Add chopped onion, salt, and pepper. Mix gently.

Transfer the mixture to the prepared baking dish and spread evenly.

Sprinkle crushed crackers evenly on top of the squash casserole.

Bake in the preheated oven for 25-30 minutes until bubbly and golden brown.

Remove from oven and let it rest for a few minutes before serving.