Pre-heat oven to 350°F. Grease an 8x8 baking dish with cooking spray or butter.
Slice the squash and zucchini into ¼-inch rounds. Set aside.
In a medium mixing bowl, mix together soup, sour cream, egg, seasoning, 1 cup cheese, and milk.
Place half the zucchini and squash in the baking dish. Pour half the sauce over the squash and zucchini. Place the remaining squash and zucchini on top. Top with remaining sauce. Top with ¾ cup cheese.
Place saltine crackers in a sealable bag place and crush. Add melted butter and toss to coat.
Evenly pour the cracker crumbs over the casserole.
Place in oven uncovered and bake for 40 minutes. Remove from oven and allow it to rest for 10-15 minutes before serving.