1. Wash zucchini and yellow squash, and trim stem and blossom ends; cut squash crosswise into 1/4-inch slices. Toss squash and onion with salt in a very large bowl. Cover vegetables with ice cubes. Cover and let stand at room temperature 3 hours.
2. Sterilize jars, and prepare lids.
3. While jars are boiling, drain vegetables, but do not rinse, discarding brine and any unmelted ice. Return drained vegetables to bowl. Combine vinegar and next 4 ingredients in a medium stainless steel saucepan. Bring to a boil over medium-high heat.
4. Pack squash and onion mixture tightly into hot jars, leaving 1/2-inch headspace. Cover vegetables with hot pickling liquid, leaving 1/2-inch headspace. Seal and process jars, processing 10 minutes.
5. Remove jars from water, and let stand, undisturbed, at room temperature 24 hours. To check seals, remove the bands, and press down on the center of each lid. If the lid doesn't move, the jar is sealed. If the lid depresses and pops up again, the jar is not sealed. Store properly sealed jars in a cool, dark place up to 1 year. Refrigerate after opening.
Total time: 4 hours, 10 min., plus 3 weeks standing time