Heat the oil in a large pan or in a wok. Fry the onions for 6-8 minutes until they are lightly browned. Add the ginger, garlic and chilli and fry for another 3 minutes. Add cumin, coriander, turmeric and chili powder. Season the chicken with salt and pepper and add to the pan. Brown the meat all around. Stir in tomatoes and water and bring to the boil, reduce the heat and simmer covered for approx. 30 minutes. Stir every now and then and add a little more water if necessary. Stir in Garam Masala and simmer openly for another 10 minutes. Garnish with the coriander leaves and serve.