From "Classic Indian Vegetarian and Grain Cooking," by Julie Sahni (William Morrow).
Wash lentils thoroughly. Place in a pot with 3 cups water and turmeric. Cook, partially covered, until very tender, about 30 minutes.
Combine lentils and 1 cup water in a deep pot. Whisk to crush some of the lentils. Add the tomatoes, cumin, ground coriander, cayenne, onion, garlic and salt and bring to a boil. Lower heat and cook, partially covered, 10 minutes. Add lemon juice.
Heat oil in a small skillet until hot. Add mustard seeds and cover so the seeds do not fly out of the pan. When they stop spattering, mix into soup. Stir in fresh coriander.
Makes 6-8 servings.