To make the spice paste, finely chop the onion and grate or chop the garlic. Peel, then grate or chop the ginger.
Add the onion, garlic and ginger to a food processor along with the chilli powder, turmeric, cumin, ground coriander and ground ginger. Blitz until completely smooth. Transfer to a bowl and set aside.
In a mixing bowl, combine the chicken thighs with the garam masala and a generous seasoning of salt.
Heat a large, high-sided pan over a medium high heat and add the sunflower oil. Once hot, fry the chicken until golden brown on all sides. Remove the chicken from the pan and set aside on a plate.
If required add another drop of oil and over a medium high heat, fry the spice paste for 6 minutes until aromatic, stirring regularly.
While the spice paste is frying off, cook your rice. Rinse the basmati rice in a sieve under cold running water. Place the rice in a small saucepan and cover with cold water. Place on a high heat until it starts to simmer, then cover with a lid and turn the heat down to low. Allow to simmer for 15-20 minutes without lifting the lid. Take the pan off the heat and allow to stand with the lid on for 5 minutes before serving.
Add the chicken back to the pan with the chopped tomatoes and chicken stock. Bring to a steady simmer and then turn the heat to low. Simmer gently for 12-20 minutes. Finish by stirring through a squeeze of lemon juice to taste.
Fluff the rice with a fork before serving with the curry and naan bread. Garnish with coriander leaves.