Use firm avocados. Peel and slice as you like, then soak in a bath of cold salted water and lime juice for about an hour. This step draws some of the oil from the avocado, making it more receptive to the pickling liquid (makes them prettier, too). Drain, dust generously with salt and red pepper flakes, and pack into jars. Fill jars with white vinegar, then pour vinegar from jars into a saucepan. For each pint of liquid, add the juice of half a lime, a teaspoon each of whole black peppers, yellow mustard seeds, and diced cilantro.