Dry-fry the cumin and coriander seeds in a small, nonstick frying pan for a few minutes until aromatic and toasted. Leave to cool, then crush together.
Mix together the crushed seeds, cayenne, chillies, turmeric, garlic, ginger and the measurement water in a large saucepan and simmer for 10 minutes until thickened. Season with salt and pepper, then stir in the coconut milk and tamarind paste.
Arrange the aubergine slices on a foil-lined grill rack and brush the tops with some of the curry sauce. Cook under a preheated hot grill until golden.
Stir the aubergine slices into the curry sauce. Serve hot with mini plain naan breads.