This pickle is made the traditional way, with a short brining to keep the onions crisp, and with cool rather than hot vinegar. If you'd like to can the onions, though, use two one-pint or four half-pint Mason jars instead of a quart jar, and pour the spiced vinegar over the onions while the vinegar is hot. Seal the jars immediately with hot two-piece caps, and process the jars for 10 minutes in a boiling-water bath. Sharp-tasting, brown malt vinegar is available at most supermarkets. If you can't find it, or if you'd like a milder pickle, use white wine vinegar.