Start by adding the rice and a little salt to boiling water in a large saucepan. Bring back to the boil, cover, then reduce to a simmer for 10-12 minutes. Remove from the heat, keep the lid on and set to one side.
Heat a large frying pan over a moderate heat until hot, then add the cumin, coriander, fenugreek and mustard seeds and toast until you can smell an aroma from the spices. Remove from the heat and grind into a powder using a pestle and mortar.
Heat the olive oil in a large casserole dish over a moderate heat and sweat the onion for 4-5 minutes until soft. Add the garlic and continue to cook, stirring occasionally for 3-4 minutes. Add the red chilli, the ground spices, the chilli powder, garam masala, sugar and 1 tsp of salt at this point, stir well, and cook for 1-2 minutes. Add the coconut milk and 100ml of water to the dish and bring to a simmer.
Add the chicken and simmer for 12-15 minutes until it is cooked through with no pink showing. Adjust the seasoning using salt, pepper and lemon juice. Re-heat the rice if necessary and spoon into individual pudding dishes. Invert the pudding bowls of rice onto serving bowls and spoon the hot curry on top of the rice.
Serve with a garnish of coriander leaves and slices of the toasted naan bread on the side.