Slice the aubergines lengthways, keeping them joined at the stem. Place in a large shallow dish, sprinkle with salt and, set aside.
Heat the oil in a kadhai or heavy-based pan over high heat. Reduce the heat, add the onions and fry for 2 minutes, or until translucent. Add the ginger and garlic pastes and stir-fry for 1 minute, then sprinkle over 1 teaspoon water.
Add the ground spices together with the tomato and season with salt. Stir-fry over medium heat for 2 minutes, then remove from the heat and allow to cool.
When cool, fill the aubergines evenly with this mixture, then place them back in the same pan, cover and simmer for 5-7 minutes, turning once or twice, until hot.