Prick the aubergines all over with a fork, then place on a roasting tray. Roast for 50 minutes to 1 hour, or until the skin blackens and chars and the flesh can be easily pierced with a spoon. Allow to cool to room temperature.
Cut the cooled aubergines in half, scoop the flesh into a food processor and pulse until smooth. Set aside.
Peel and finely chop the onion and garlic, trim and finely slice the chillies, and peel and finely grate the ginger (you should end up with roughly 2 teaspoons). Roughly chop 3 of the tomatoes and slice the remaining, then pick and finely chop the coriander leaves.
Heat the oil in a large frying pan. Add the onion and sauté for 5 to 6 minutes, or until softened.
Add the garlic, ginger and chilli, then stir-fry for 1 to 2 minutes. Stir in the tomatoes and curry powder and cook for a further 12 to 15 minutes, or until softened.
Next, add the reserved aubergine and cook, for 3 to 4 minutes, stirring regularly. Stir in the coriander, then remove from the heat.
Garnish with the extra tomato slices, and serve with the oven-baked corn tortillas.