In a medium saucepan, heat the vegetable oil. Add the mustard seeds, cumin, peppercorns and fenugreek and cook over moderate heat until the mustard seeds start to pop. Stir in the curry leaves, dried chiles and asafetida. Add the dhal to the pan along with the tomato, onion, salt, tamarind liquid and the remaining 6 cups of cold water. Bring to a boil over high heat. Cover the saucepan and simmer the mixture over low heat for 30 minutes. Stir, then allow the solids to settle. Using a ladle, serve the hot broth in large bowls or mugs.