You need: 4-5 large tomatoes, rasam podi, 1/2 tsp haldi (tumeric), salt to taste. For tempering (tadka): 1/2 tsp mustard seeds (sarson/kadagu), 3-4 curry leaves (kadipatta/karuvapilai), 1/2 tsp cumin seeds (jeera/jeeragam), 1 tsp ghee.
How to: Chop the tomatoes and boil them in 4-5 cups water until soft. Mash them well. Add salt, turmeric, and 1 heaped tsp rasam podi, mix well, and bring to a boil. Let it boil for 2 minutes. Turn off the heat. Heat the ghee in a small pan. Add all the tempering spices and let them sputter. Pour into the rasam and stir well. Eat with rice and dal.