Combine the first six ingredients in a large saucepan and bring to a boil.
Toss the onions and the zucchini in a glass or plastic bowl, and pour the boiling liquid over them. Let it stand for an hour, stirring now and then.
Return everything to the saucepan and bring to a light simmer for three minutes. Leaving the pot at a simmer, start packing the mixture into sterilized canning jars. Fill to within a half-inch of the top, making sure that the vinegar mixture covers the vegetables completely.
Cap each jar with a new cap at once, and process the jars for five minutes in a boiling-water bath.