Cut the aubergines into slices, sprinkle with salt and let sit for a while. Rinse and pat dry. Brush the slices with oil on both sides, place on a baking sheet and bake in a hot oven. When the top is brown, turn the slices and bake from the other side until they are brown and soft.
In the meantime, peel the garlic, chop it finely and mix it with the yogurt. Stir the mint and thinly sliced onions into the yoghurt and season with salt, pepper and sugar.
Layer the aubergine slices alternately with the yogurt sauce in a bowl. They should stand in a cool place and last half a day.