Chop the eggplants into bite-sized pieces and put in a glass or ceramic bowl. Sprinkle with the salt. Leave for about 4 hours, or as long as possible, then squeeze out any excess water.
In a mortar and pestle, pound the mustard seeds, ginger and garlic to a rough paste. Add the turmeric and pound again.
Heat the oil in a saucepan and fry the spice mixture for a minute. Add the eggplant, chilli, vinegar, sugar and salt. Cook over low heat until the pickles are dry and the oil splits and sits on top. This will take maybe 45 minutes. Taste for seasoning. Let cool, then store in sterilised jars. The pickles will last for a few weeks if stored in the fridge.