2. Bring the water to a boil in a medium saucepan. Add the chopped tomato, the ginger, cilantro, and green chile. Turn the heat down and simmer 3 minutes.
3. Add the tomato-garlic puree, the curry or rasam powder, cumin, black pepper, and salt and simmer 5 minutes.
4. For the tempering oil, combine the oil and mustard seeds in a small frying pan or saucepan over medium-high heat. Cover (the mustard seeds splatter and pop) and cook until you hear the mustard seeds crackle, 1 to 2 minutes.
5. Add the chiles and cook uncovered, stirring, until they start to brown, about 30 seconds.
6. Add the curry leaves and coconut, if using, and cook, stirring, until the coconut turns golden brown, about 30 more seconds.
7. Pour immediately into the soup and stir. Serve hot.
Adam Rapoport, editor in chief of Bon Appetit magazine and the website www.bonappetit.com, knows his way around a grill. He has edited an entire book on the subject: The Grilling Book: The Definitive Guide from Bon Appetit.