Preheat the oven to 350°F and lightly grease a 9x13-inch baking dish with butter or cooking spray.
In a large pot of boiling salted water, cook the sliced squash and chopped onion for 8-10 minutes, or until tender. Drain well and transfer to a large mixing bowl.
To the bowl, add the grated cheddar cheese, sour cream, melted butter, beaten egg, salt, pepper, and garlic powder. Stir until all ingredients are evenly combined.
Spread the squash mixture evenly into the prepared baking dish and smooth the top with a spatula.
Sprinkle the crushed crackers evenly over the top of the casserole.
Bake in the preheated oven for 25-30 minutes, or until the top is golden brown and the casserole is bubbling around the edges. Let cool for 5 minutes before serving.