In a large bowl wash the lentils in several changes of cold water until the water runs clear and drain them in a fine sieve. In a large heavy saucepan cook the onions in the oil over moderate heat, stirring, until they are softened, add the garlic and the gingerroot, and cook the mixture, stirring, for 1 minutes. Add the cumin and the ground coriander seeds and cook the mixture over moderately low heat, stirring, for 1 minute. Add the lentils, the broth, 1 1/2 cups water, and the tomatoes and simmer the mixture, covered, for 15 to 20 minutes, or until the lentils are tender. In a blender or food processor purée the mixture in batches, transferring it to a bowl as it is puréed, return the soup to the pan, cleaned, and season it with salt and pepper. The soup may be made 3 days in advance and kept covered and chilled. Reheat the soup and thin it, if necessary, with water.