Cut zucchini into 1/4-inchthick slices. Place zucchini in a 12- x 8- x 2-inch baking dish. Sprinkle with salt and pepper.
Layer half each of tomato and onion over zucchini; sprinkle each layer with salt and pepper. Dot each layer with butter. Repeat procedure, using remaining vegetables.
Sprinkle cracker crumbs over casserole. Bake at 350° for 40 minutes or until vegetables are tender.