CPut ginger and garlic into a food processor and add 1/2 cup water. Blend until fairly smooth. Heat the oil in a wide, heavy saucepan over a medium flame. When hot put in the cumin seeds. Stir for 1/2 a minute. Pour in the ginger-garlic paste. Stir and cook for about 2 minutes. Put in the coriander and stir a few times. Put in the chopped tomatoes. Stir and cook for 2 minutes while mashing the tomato pieces with the back of a slotted spoon. Put int the beans, add salt and the cup of water and simmer them. COver, turn heat to low and cook for 8-10 minutes or until the beans are tender. Remove the cover. Add the lemon juice and a generous amount of freshly ground pepper. Turn up the heat and boil away the remaining liquid stirring the beans gently as you do so.