Peel and finely grate the garlic and ginger, and deseed and finely chop the chillies, then add to a bowl. Peel and finely slice the onions.
Heat a good splash of oil in a large pan over a medium heat, add the mustard seeds. When they start to pop, add them to the ginger and garlic mixture along with the paprika, cumin, ground coriander and 2 tablespoons of garam masala.
Put half the mixture into a clean bowl with the yoghurt. Roughly chop and stir in the chicken, then leave to marinate for at least 30 minutes, preferably longer.
Place the butter in the empty pan over a medium heat, add the onions and the remaining spice mix, then cook gently for 15 minutes, or until softened – it should start to smell fantastic!
Add the tomato purée, the ground nuts, 500ml of water and a good pinch of sea salt. Stir well and simmer gently for a few minutes, or until slightly thickened slightly, then remove from the heat.
Sear the marinated chicken on a hot griddle pan or barbecue and until golden and cooked through – you can also do this under the grill, if you like.
Warm the sauce, then add the cream and remaining garam masala, then taste and correct the seasoning, if needed. As soon as it boils, take off the heat and add the grilled chicken.
Pick, roughly chop and scatter over the coriander leaves, then squeeze over the lime juice, to taste.