Preheat your oven to 350°F.
In a large skillet, melt the butter over medium heat. Add the sliced squash and chopped onion. Cook until just tender but not soft, about 7-10 minutes.
Drain any excess liquid from the cooked squash and onions.
In a large bowl, combine the cooked squash and onions, beaten eggs, sour cream, milk, shredded cheddar cheese, salt, and black pepper. Mix well.
Transfer the mixture to a greased 9x13-inch baking dish.
Sprinkle the crushed buttery round crackers evenly over the top.
Bake in the preheated oven for 35-45 minutes, or until the top is golden brown and the casserole is bubbling.
Let the casserole cool for a few minutes before serving.