1. Pre-heat the oven to 180C, 350F, Gas Mark 4 . Using a sharp knife, slit the aubergines lengthwise on one side only, just making a pocket and making sure you don't cut all the way through.
2. Heat 4 tablespoons of the olive oil in a large frying pan, add the whole aubergines and fry on all sides, adding a little more oil to the frying pan if necessary. Remove from the pan and arrange, side by side, slit upwards in a shallow ovenproof dish. Season with salt and pepper and set aside.
3. Add the rest of the olive oil to the frying pan add the onions and garlic and saut until they are slightly golden.
4. Add the tomatoes, 150ml/5fl.oz.of water, salt, pepper, oregano and sugar. Mix well, reduce the heat, cover and simmer for 15 minutes.
5. Divide the onion mixture between the aubergines, forcing the mixture into the slits by opening them slightly then bake for 40 minutes, basting once during the cooking period. Serve hot.