(Classic Indian Vegetarian and Grain Cooking)
Blanch, peel, and cut tomatoes in half. Scoop out pulp and seeds and mince or blenderize these. Set the tomato shells aside.
Put cooked lentils into a deep pot. Add 1 cup water and whisk. Add pureed tomato pulp and seeds, cumin, coriander, cayenne, onion, garlic, and salt and bring to a boil. Lower heat and cook at a gentle boil, partially-covered for 10 minutes. Add lemon juice and tomato halves and continue cooking for 1 more minute or so, until the tomatoes are heated and barely cooked. Turn off heat, cover pot and make the spice-perfumed butter.
When the spice-perfumed butter is done, pour it all into the into the soup and mix well. If you're using cilantro instead of curry leaves in the spice-perfumed butter, add it now. This should be a rather thin soup. Dilute with some water if it's too thick. Serve hot.
Pick over the lentils (or split peas or mung beans) to remove any bad beans and foreign particles. Wash and rinse several times until water runs clear, not cloudy, through them.
Put lentils in deep pot with water and turmeric and bring to a boil. Lower heat to medium and cook, partially-covered, stirring often. Leave yellow lentils and yellow split peas in for about 40 minutes; red lentils or yellow mung beans for about 25 minutes. When you've reached the appropriate time, reduce the heat, cover and continue cooking for an additional 20-25 minutes for yellow lentils or yellow split peas; 10 minutes for red lentils or yellow mung beans, or until soft. At the end there should be 4 1/2 cups lentil mush. If not, add more water to bring it to that amount. You can whisk it to make the consistency even smoother.
You will have made more than the broth recipe calls for. Set the excess aside and find some other use for it. I like it warmed for breakfast.
Measure out the spices in separate piles and have them handy as you begin to heat the ghee/oil in a small frying pan until hot. Add mustard seeds carefully and keep a lid ready as these will probably splatter like crazy. When splattering subsides add curry leaves (not the cilantro yet) and turn off heat. Pick up pan and shake for a few seconds, like you would popcorn. Pour the entire contents into the soup and mix well. Add the cilantro into the soup at this point, if you're using cilantro instead of curry leaves.