Cut eggplants in half lengthwise, then slice 5 mm (1/4 inch) thick. Spinkle slices liberally with salt (stack if necessary) and leave for 30 minutes. Rinse and dry with paper towels.
Heat half the oil in a large frying pan and fry eggplant until golden brown on each side. Drain on paper towels. Add more oil to pan as required.
Blend yoghurt with salt to taste and the crushed garlic. Place a layer of cooled eggplant in serving dish, overlapping slices a little. Season with pepper and spread some yogurt on top. Repeat, finishing with a layer of yoghurt. Cover and chill. Serve garnished with chopped walnuts if desired.