Combine first 7 ingredients in saucepan; bring to a boil. Pour over zucchini and onions; let stand 45 minutes, stirring occasionally.
In saucepan, bring mixture to a boil, then simmer 3 minutes.
Continue simmering while quickly packing one clean, hot jar (pint size) at a time. Fill to within 1/2 inch of top making sure vinegar solution covers vegetables. Cap each jar at once.
Process 5 minutes in boiling-water bath. You may also follow canning instructions that come with a Mason kit.
Makes 6-7 pints.