First, heat a large, heavy-based pan to a low heat. Lightly toast the cumin and nigella seeds for around a minute, until aromatic. Remove from the heat and set aside.
Next, add the coconut oil to the pan and turn the heat up to medium. Add one of the onions, keeping the other half to one side. Cook for around 5 minutes until soft.
Meanwhile, add the toasted spices to the food processor along with the remaining onion, olive oil, curry powder, turmeric, garlic, coriander, ginger, and the flesh from one of the mangoes. Whizz together to form a paste.
Then, add the curry paste to the pan and gently simmer for 3-4 minutes to really release the flavours.
Meanwhile, shred the meat from the roast chicken, using as much as you can from all of the bird.
Pour the coconut milk and chicken stock into the pan, and then add the chicken. Stir well, and leave to simmer for 15 minutes.
Finally, a couple of minutes before serving, dice the remaining mango and add to the pan. Stir well, and serve garnished with fresh coriander.