In a medium saucepan, combine the vinegar, sugar, cinnamon stick, cloves, allspice, chile, bay leaves and peppercorns. Bring to a boil over high heat.
While the brine is heating, peel and trim the onions. Slice them into rings about 3/8” thick. Separate each slice into its individual rings, discarding any thin, leathery outer rings. When the brine mixture boils, add about 1/3 of the onion rings and stir them under. They will turn hot pink almost immediately. As soon as the brine begins to simmer around the edges, about 20 seconds, stir them under again, and then remove the pot from the heat.
Remove the onions with a slotted spoon and spread them on a platter or rimmed baking sheet to cool. They should still be firm. Repeat with the remaining onions, in two batches.
Once the onions have cooled—you can slip them into the fridge to speed them along—repeat the entire process, again in three batches, two more times, always adding the onions to boiling brine, retrieving them promptly when the brine begins to simmer again, and cooling them completely (if you are cooling your onions in the fridge, this will not take as long as you think. It’s not so bad).
After the third round of blanching, thoroughly chill the brine. Transfer the onions and brine into jars (we used two quart-size Mason jars, which were each about two-thirds full). The most important thing is that the onions be in a container that allows them to remain submerged in the brine. Store in the refrigerator. Age the pickles for at least a day before serving. They’re very good after 24 hours, but the flavors will have melded more harmoniously after 48. From there out, it’s delicious all the way.