Grind the nigella seeds, fennel seeds, cumin seeds and mustard seeds in a pestle and mortar or spice grinder until fine.

Prepare the marination: mix together all the marination ingredients and set aside for at least 4 hours and a maximum of 24 hours.

When ready to start cooking, add the oil into a deep pan or pot. Once heated, add in the diced onions and sauté them over a medium to high heat until the onions turn brown.

Add in the marinated chicken. Sauté this until the chicken no longer remains pink and any water released from the yogurt concentrates. It'll look pretty dry when this step is completed. Keep stirring over a medium to high heat to ensure it doesn't burn.

Add the tomato concentrate/tomatoes. If using tomato concentrate, also add ½ cup/125mls water too. Cover and simmer everything over a low heat for 25 minutes.

As the chicken is cooking, prepare the green chillies. Mix the lemon juice and chaat masala, then set aside. Use a knife to create a slit down the centre of the chilli, then apply pressure to the top and bottom to open up the slit. Insert 2-3 teaspoon of the lemon and chaat masala mixture into the chillies, then set aside.

Once the cook time for the chicken is up, uncover and add in the chillies. Allow the chillies to cook alongside everything for 5 minutes.