How to Make It
Peel and mince the tomatoes. Heat the oil in a medium saucepan. Add the mustard seeds, cover the pan and cook over high heat until the seeds begin to pop. Reduce the heat to moderately low. When the popping stops, add the onion and cook, uncovered, stirring until softened, about 5 minutes. Add the cumin, coriander, turmeric and chiles to the saucepan and cook, stirring, until fragrant, about 1 minute.
Stir in the red lentils, tomatoes and water; season with salt and bring to a boil. Simmer the soup over moderately low heat until the lentils have fallen apart, about 45 minutes. Ladle the tomato soup into bowls and serve.