Heat the oil in a pan and fry the onion and garlic over a low to medium heat for 4-5 minutes, or until softened. Mix the chilli powder, turmeric and salt together in a bowl until well combined, then add the chicken pieces and mix well to coat. Add the coated chicken to the pan and fry for 10-15 minutes, or until golden-brown and cooked through. Stir the tomatoes, ginger, cumin and coriander into the pan, then reduce the heat until the mixture is just simmering. Cover the pan with a lid and cook for 20-30 minutes, stirring occasionally and adding a tablespoon of water every so often if the mixture appears too dry. Stir in the butter and add the lemon juice, to taste. To serve, spoon the chicken jalfrezi onto serving plates. Serve with rice, naan and lime wedges on the side, then garnish with the chopped coriander and sliced red chillies.