1. Meanwhile, heat the oil and butter in a non-stick saucepan. Add the onion, season lightly and fry until soft and golden at the edges.
2. Meanwhile, blend the tomatoes and cashews until smooth, you can add a little water if it seems dry.
3. Add garlic to the onions to the pan and cook gently for 40-50 seconds or until the garlic smells cooked.
4. Add the blended tomatoes and nuts and a splash of water, turn up the heat and cook until thick and little oil droplets release back into the pan- about 6-8 minutes.
5. Add the chicken, butter chicken paste and tomato purée and 150ml water and bring to a simmer and cook until the chicken is cooked through, 7-9 minutes.
6. Add extra boiling water if it is too thick. Stir in the double cream, taste and adjust seasoning.
7. Finish with the coriander and serve.

For added step

Stir together the paste and the yoghurt and season lightly. Add in the whole chicken fillets (ideally at room temperature) and coat well. Place on an oiled baking tray and grill for 8 minutes or until the upper surface is lightly charred in places. Tear or chop into large pieces and add to the curry as above, but with less water and only cook for 3-4 minutes or until cooked through.

TO SERVE:

• Naan with melted butter and finely chopped coriander
• Red pickled onions on the side
• Garnish with finely chopped coriander