Combine the vinegar, water, sugar, salt, and pickling spice in a small saucepan and bring to a boil.
Stir to dissolve the salt and sugar.
Remove the pan from the heat and let the mixture cool to room temperature.
Halve, pit, and peel the avocados, then cut them into 1/4" thick slices.
Peel and slice the onion into 1/4" thick wedges.
Place the avocado and onion slices into a pint-size glass jar.
Pour the cooled brine into the jar and cover.
Gently turn the jar upside down and then upright again to distribute the brine evenly.
9. Â Refrigerate for 2 hours.
The pickled avocado slices will keep, tightly sealed in the refrigerator, for up to 1 week.