Add the chicken to a large bowl and add a tsp salt, the spices, and a drizzle of oil. Toss to thoroughly coat the chicken and set aside.
Heat a heavy based pot over high heat and fry the chicken pieces until golden on both sides, making sure not to overcrowd the pan - cook in batches if required.
Remove the chicken and set aside.
Reduce the heat to medium high and add a drizzle of oil to the pan.
Add the onions, garlic, ginger and coriander roots, and sauté for a few minutes until softened and translucent.
Add back the chicken and all of it's juices, followed by the tinned tomatoes and coconut milk.
Leaver to simmer for 15-20 minutes until thick and glossy.
Optionally, add the corn starch slurry and simmer for another minute or 2 until further thickened.
Transfer the curry to a bowl and set aside in the fridge until cooled completely.