In a saucepan, add the vinegar, water, sugar, salt and black peppercorns and bring to a boil. Add the chili and garlic, and simmer gently for 5 minutes, then leave to cool.
Cut avocados in half, remove pits and slice them thinly. Place them in a Mason jar, along with the coriander stalks. Pour the cooled pickling lid over them, seal tightly with a lid and put into the fridge. Chill for 24 hours then serve.