Preheat the oven to 350°F. Grease a 2-quart casserole dish.
Heat the oil in a large skillet over medium heat. Sauté the squash, onion, and butter until soft. Transfer to a bowl and stir in the Parmesan, Cheddar, and sour cream. Add salt and pepper, to taste.
Place in the prepared casserole dish and sprinkle the cracker crumbs evenly over the top. Bake for 20 minutes or until the top is golden and bubbly.