Scoop the onions into a glass or stainless bowl. Cover with very hot tap water. (The water should be steaming; if yours isn’t heat it a little on the stove.) Let stand 20 minutes, then drain. Sprinkle with 1 teaspoon salt. Cover with the lime juice and lay a plate on top to keep them submerged. Cover the bowl and refrigerate 3 or 4 hours (or, preferably, overnight). Scoop the onions into a storage container, cover and store in the refrigerator for up to a month or more. After a few weeks, the texture of the onions will begin to deteriorate.
Variations on the theme: As with the recipe here some habanero can be pickled with these onions, though it’s not that common in the Yucatan with an habanero salsa would be served alongside. A little Mexican or Yucatecan oregano could be added, and I sometimes like a little black pepper—but that’s just me.