Put the chicken in a bowl or plastic container and toss with the lemon juice, a large pinch of salt, then set aside while you make the marinade. Blitz the ginger, garlic, chillies and spices to make a paste, adding a splash of water to loosen if needed. Set aside half of the paste for the sauce. Tip the yoghurt into a food processor with the rest of the paste and blitz until smooth. Pour over the chicken and marinate for at least 4 hrs (overnight, or up to 48 hrs is best if you have time).

While the chicken is marinating, make the sauce. Heat the butter in a casserole dish or sturdy saucepan with a lid. Add the onions along with a pinch of salt and gently cook for 10 mins, until golden. Add the reserved paste and continue to cook for 3-4 mins until starting to colour. Stir in the tomato purée and cook for another 3-4 mins until everything is sticking together, then add the chopped tomatoes, 2 cans of water (if cooking for 4) and the stock. Simmer for 20 mins to make a thick, glossy sauce. Leave to cool, then cover and keep chilled. Will keep chilled for up to three days, or frozen for three months.

To cook the chicken, fire up the barbecue and wait for the coals to turn orange and silvery grey - you can also do this under a hot grill or on a griddle pan. Be sure to turn on the extractor fan. Lay the chicken pieces flat on the barbecue bars and cook for 5-8 mins on each side until nicely charred and just cooked through. Use tongs to lift the chicken onto a board, and when just cool enough to handle, chop into large chunks, then stir into the sauce along with any juices from the board. Place the pan on the side of the barbecue to simmer gently for 15 mins until the chicken is coated in the rich sauce. Season to taste. If your barbecue has a lid, simmer everything with the lid on for an extra-smoky flavour. Serve with a scattering of chopped coriander and sliced chilli, if using, alongside cooked basmati rice and flatbreads or naans warmed on the barbecue.