Cut the aubergine lengthways into 5 mm (¼ inch) thick slices. Mix 3 tablespoons of the oil with the cumin and salt and pepper and brush all over the aubergine slices. Cook in a preheated ridged griddle pan or under a preheated hot grill for 3–4 minutes on each side until charred and tender. Leave to cool, then finely chop.
Mix the aubergine into the yogurt in a bowl, then stir in the garlic, coriander, lemon juice, the remaining oil and salt and pepper to taste. Transfer to a serving bowl.
Cook the tortillas in the preheated griddle pan or under the preheated hot grill for 3 minutes on each side until toasted. Cut into triangles and serve immediately with the aubergine dip.