In a large skillet, saute the summer squash, zucchini, mushrooms and onion in oil until tender; drain., In a large bowl, combine the vegetable mixture, cheese, soup, sour cream and salt. Transfer to a greased 11x7-in. baking dish. Combine cracker crumbs and butter. Sprinkle over vegetable mixture. , Bake, uncovered, at 350° for 25-30 minutes or until bubbly.