Photograph: Tara Fisher
Light the barbecue. In a food processor blend together the marinade ingredients with a generous teaspoon of freshly ground black pepper and the lemon juice, to taste – how much you use will depend on how tart the yogurt is – until smooth. Season really well as the prawns will absorb some of the salt. Tip the prawns into the marinade, toss to coat and leave to marinate for 40 minutes. Meanwhile, soak about 15 wooden skewers in a bowl of water.
To make the salsa, heat the oil in a medium pan and add the nigella seeds, if using. Cook for 30 seconds then add all of the remaining ingredients, apart from the coriander. Season and cook for 2-3 minutes or until the tomatoes have softened. Remove from the heat and stir in the chopped coriander.
Skewer your prawns, about 3-4 per skewer. Barbecue for around 2-3 minutes each side or until cooked and lightly charred. Barbecue the lime halves, too. Warm the naans on the barbecue for a minute on each side.
Split each naan to make a pocket and fill with lettuce, prawns, tomato salsa and crème fraîche. Serve immediately with lime halves to squeeze over.
Get ahead: make the tomato salsa the day before; chill, but serve at room temperature. Make the marinade a day ahead; chill, and add the prawns 45 minutes before cooking.