Use avocados that are not quite ripe in this recipe; pickling softens them.
Whisk together vinegar, 1/2 cup water, salt, mustard seeds, sugar, peppercorns, and chile de arbol, if desired, in a medium nonreactive pot. Bring mixture to a simmer over medium heat, stirring until sugar and salt dissolve, then let it cool to room temperature, about 20 minutes.
Place avocado slices in a 1-pint jar and top with the cooled brine. Refrigerate for at least 1 hour or up to 1 month.
Adapted from An Avocado a Day: More Than 70 Recipes for Enjoying Nature’s Most Delicious Superfood. Copyright 2016 by Lara Ferroni. All rights reserved. By permission of Sasquatch Books.