Combine zucchini, onion and salt; let stand overnight. Rinse and drain well. Place in a large kettle or Dutch oven along with remaining ingredients; cook until mixture thickens, stiring constantly. Do not overcook. Pack hot into hot sterilized jars, leaving 1/4 inch headspace. Adjust caps. Process 10 minutes in a boiling water bath. Yield: 16 half-pints.