Cut the aubergines in half lengthways and scrape out most of the the flesh, leaving a wall about 5mm (1/4 in) thick all the way round. Sprinkle with the salt and set aside. Chop the flesh.
To make the stuffing, heat the oil and saute the onion and aubergine flesh. Add the mince and garlic and cook until well browned. Add the remaining ingredients, cover and simmer for 45 mins.
Preheat the overn to 190C/375F/Gas mark 5. Rinse and dry the aubergine shells and set them in a baking dish. Fill with the mince and cook for 45 mins.
Meanwhile, make a white sauce with the butter, flour and milk. Simmer for 1 min until thickened and smooth. Remove from the heat, whisk in the yogurt and egg yolk. Spoon over the aubergines and return to the overn for 15 mins. Drizzle over any cooking juices and serve with a spinach salad and new potatoes, if wished.