Trim the green beans and cut them crosswise at 1/4 in intervals. Put the ginger and garlic into the container of an electric blender or food processor. Add 1/3 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.