To make ground, roasted cumin: Toast a teaspoon of whole cumin seeds in a small skillet over medium heat. Cook until the cumin seeds have turned a few shades darker and are smelling fragrant. Grind in spice grinder or mortar and pestle.
Trim the beans and cut them crosswise into 1/2-inch lengths.Put the ginger and garlic into the container of an electric blender or food processor. Add 1/2 cup of water and blend until fairly smooth.Put the oil in a wide, heavy pot over medium heat. When hot, add the cumin seeds. Five seconds later, add the crushed red pepper flakes. As soon as it darkens, pour in the ginger-garlic paste. Stir and cook for about a minute. Add the coriander. Stir a few times. Add 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.Put in the beans, salt (maybe not all of it to start), and the remaining water. Bring to a simmer. Cover, turn heat to low, and cook for 8 to 10 minutes or until the beans are tender. Remove the cover and add the juice of half a lemon, the ground roasted cumin seeds, and a generous amount of freshly ground black pepper.Taste and adjust with more lemon, salt, and/or pepper as necessary.