Slice the chicken into thin pieces and place in a bowl along with 1 tsp chilli powder, 1 tbsp ginger paste, 1 tbsp garlic paste and a pinch of salt and mix well. Set aside for 15-20 minutes whilst you prepare the other ingredients
Roughly chop the onion and do the same to the tomatoes. Don't worry too much about chopping finely everything is going into a food processor a bit later!
After 15-20 minutes has passed, put 1 tbsp of oil into a large sauté pan or wok and fry the chicken until it has taken on some colour and is almost cooked through. It will be finished later in the sauce
Remove the chicken from the pan and set aside. Add the remaining tablespoon of oil and, when hot, add the onions and a knob of the butter and sweat them down until they are soft but not coloured
Add the chopped tomatoes, the cashew nuts and the water. Next add the remaining ginger paste, garlic paste, a pinch of salt, the sugar, the garam masala, the vinegar and the remaining chilli powder. Mix well and cook for at least 20 minutes until the whole mixture has cooked and turned into a slightly mushy consistency
Allow the mixture to cool slightly and then pour into a food processor or, if you prefer, a bowl to be blitzed with a hand blender. Blitz the mixture until smooth and then pass it through a sieve into a clean pan
Add the remaining butter, a tablespoon of cream and the chicken. Mix well and then add the fenugreek leaves. Simmer for 5 minutes or so and then serve drizzled with a little of the remaining cream and the chopped coriander