Photograph: Tara Fisher
Blend the marinade ingredients together with a generous teaspoon of freshly ground black pepper until smooth. Add the lamb and toss to coat. Cover and leave for as long as possible – overnight in the fridge is best. Bring back to room temperature before cooking.
Stir together the ingredients for the herb yogurt and season with salt and lots of freshly ground black pepper; set aside. Put the sliced red onion in the vinegar with a pinch of salt and set aside to soften for at least 30 minutes. Meanwhile, put about 15 wooden skewers into a bowl of water to soak.
To make the tamarind chutney, toast the cumin in a dry pan until aromatic, then grind in a food processor or bash to a powder with a pestle and mortar. Tip into a small nonstick pan and heat together with the remaining ingredients, 150ml of water and teaspoon each of salt and freshly ground black pepper. Cook, uncovered, until syrupy – around 5-6 minutes. Remove from the heat, taste and adjust the seasoning if necessary; it should be sweet and sharp and have enough salt to balance.
Light the barbecue. Season the lamb with salt, then skewer tightly, 4-5 pieces per skewer, and cook for 6-8 minutes, turning halfway, or until done to your liking. Warm the naans on the barbecue for a minute on each side.
Split your naans to make pockets, then stuff with lettuce, tomato and cucumber slices, red onion and a tablespoon or two of the herb yogurt. Add the hot lamb and drizzle over 1-2 teaspoons of the tamarind chutney. Serve immediately.
Get ahead: marinate the lamb and make the tamarind chutney up to 24 hours in advance. Make the herb yogurt a few hours ahead; chill.