Wipe the aubergine, prick it all over with a fork and cook on a hot barbecue for 20—25 minutes, turning frequently, until the flesh feels soft and the skin is blackened. Set aside until cool enough to handle.
Peel and discard the skin, then mash the flesh and strain in a sieve to extract all the excess liquid. Transfer the flesh to a blender or food processor, add the garlic, lemon juice, coriander, tahini, cayenne and some salt and pepper and blend until fairly smooth. Transfer to a bowl, cover and leave to marinate for several hours for the flavours to develop.
Brush the tortillas lightly with a little oil and toast on the barbecue for 1 minute on each side. Cut into strips and serve with the dip.