Preheat oven to 375°F (190°C). Lightly grease a 9x13 inch baking dish.
In a large bowl, combine the sliced yellow squash, sour cream, milk, shredded cheddar cheese, chopped onion, salt, and pepper. Stir until well combined.
Pour the squash mixture into the prepared baking dish, spreading it evenly.
In a small bowl, combine the crushed crackers and melted butter. Sprinkle this mixture evenly over the top of the casserole.
Bake for 30 minutes, or until the casserole is bubbly and the topping is golden brown.
Let stand for a few minutes before serving.
Notes
This casserole is a wonderful side dish for a variety of meals.