Trim the green beans and cut them crosswise at quarter in intervals. Put the ginger and garlic into the container of an electric blender or food processor. Add one third of the water and blend until fairly smooth. Heat the oil in a wide, heavy saucepan over a medium flame. When hot, put in the cumin seeds. Five seconds later, put in the crushed chili. As soon as it darkens, pour in the ginger-garlic paste. Stir and cook for about a minute. Put in the coriander. Stir a few times. Now put in the chopped tomatoes. Stir and cook for about 2 minutes, mashing up the tomato pieces with the back of a slotted spoon as you do so. Put in the beans, salt and the remaining water. Bring to simmer. Cover, turn heat to low and cook for about 8-10 minutes or until the beans are tender. Remove the cover. Add the lemon juice, roasted cumin, and a generous amount of freshly ground pepper. Turn heat up and boil away all of the liquid, stirring the beans gently as you do so.