Preheat the oven to 350 degrees. Grease a 2 quart casserole dish. Heat the oil in a large skillet over medium heat. Saute the squash, tomatoes, 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.