Cut the chicken into bite-size pieces and mix with the lemon juice, and garam masala and season with salt.
Heat the oil or ghee in a deep frying pan or saucepan over a medium heat and cook the onion for 6-7 minutes until softened and becoming golden. Add the chicken and fry for 3-4 minutes until it has become opaque in colour. Stir in the Madras paste and cook for a couple of minutes before adding the chopped tomatoes and coconut. Cover with a lid and leave to simmer gently for about 20 minutes. Stir in the chopped coriander and serve straight away with naan bread.