1. Halve your avocado(s), slice them lengthwise, and scoop the slices out of the skin with a spoon. Divide the slices into airtight jars.
2. Whisk all brine ingredients together in a medium nonreactive pot. Bring to a simmer over medium heat, stirring until sugar and salt dissolve. Once the mixture has cooled to room temperature, divide the liquid between your jars.
3. Seal the jars and let them sit in the fridge for about two hours (and not much more, or they'll get overpowered by the vinegar).
And there you have it. Your unusable avocado is transformed into something tangy, tart, and tasty in a matter of hours.
Though they do soften during this process, we wouldn't recommend mashing your avocado pickles into guacamole or onto toast—they're a little too harsh in that large a scale. But other than that, you can pretty much use 'em anywhere you'd use pickles or avocados: atop a salad, in a grain bowl, in tacos, in soup, or even in a sandwich.
So next time—and there will inevitably be a next time—you buy unripe avocados thinking you'll save them for later and then realize you need them immediately, consider slipping the slices into a quick brine. Who knows: You might end up like me and start shopping specifically for rock-hard avocados to pickle.