Preheat the oven to 375°F (190°C) and grease a 9x13 inch baking dish.

In a large pot, boil water and cook the sliced squash for 8-10 minutes until fork-tender. Drain well.

In a mixing bowl, combine the drained squash, melted butter, sour cream, cheddar cheese, and chopped onion. Season with salt and pepper. Mix until well blended.

Transfer the mixture to the prepared baking dish and smooth it out evenly.

Sprinkle crushed crackers evenly over the top of the squash mixture.

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

Let the casserole rest for about 5-10 minutes before serving.