Whisk together vinegar, the 1/2 cup water, salt, mustard seeds, sugar, peppercorns, and chile de Arbol if you want a little spiciness, in a medium nonreactive pot. Bring the mixture to a simmer on medium heat while stirring until the sugar and salt dissolve. Let the mixture cool to room temperature, about 20 minutes.

Place the avocado slices in a 1-pint jar and top them with the cooled brine. Refrigerate for at least 1 hour before you eat 'em, though you can leave them in there for up to a month if you're in no rush.