Heat the oil in a large saucepan over moderate heat. Fry the onions, ginger and garlic until the onions are translucent. Add the tomato and stir-fry, breaking it up with the spoon. Pour in 300 ml / 1/2 UK pint water, and stir in the other ingredients. Bring to the boil. Lower heat, cover and simmer for 30 minutes. Remove from heat and allow to cool. Pour into a blender or food processor and liquidize. (Note: this is only one formula of thousands possible. Theres no more a single recipe for Balti sauce than theres a single recipe for chili. Some cooks like more garlic, some more chili powder; some use fresh chilies as well: some add yogurt, coconut, cream...you name it.) (Adapted from 100 BEST BALTI CURRIES, Lowe & Davidson)