For the karahi curry sauce: heat the oil in a pan, add the onions, garlic and chillies and fry over a medium heat for approximately 10 minutes or until the onions are caramelised.
Add the cumin, coriander, turmeric, salt and garam masala and stir. Mix in the ginger and pureed tomatoes and cook for 1 minute, stirring. Add the water and cook for about 5 minutes or until the oil surfaces. Remove from the heat and keep warm.
For the chicken: heat the oil in a wok or heavy-based frying pan and add the chicken. Stir-fry over a medium heat for 8-10 minutes or until the chicken is golden brown all over. Stir in the karahi curry sauce and simmer for 2-3 minutes. Garnish with cilantro leaves.