How to Make It
In a small saucepan, combine the dhal with 1 1/4 cups of water and 1/8 teaspoon of the turmeric and bring to a boil. Cover and cook over low heat until soft, about 30 minutes (45 minutes for yellow split peas). Partially mash the dhal with a potato masher.
Meanwhile, in a large saucepan, heat 1 tablespoon of the oil. Add the fenugreek and asafetida and cook over moderately high heat until fragrant and lightly colored, about 1 minute. Add the okra, potato, tomato, onion, cilantro, serrano chile, coriander, salt, cumin, the remaining 1/8 teaspoon of turmeric, dissolved tamarind, cayenne and black pepper along with the remaining 2 cups of water and bring to a boil. Reduce the heat to moderately low and simmer until the potatoes are tender, about 18 minutes. Add the mashed dhal and simmer 10 minutes longer.
In a small skillet, heat the remaining 1 tablespoon of oil. Add the mustard seeds, curry leaves and dried red chile. Cover and cook over moderately high heat until the mustard seeds have popped for a few seconds, then stir into the vegetable and dhal mixture. Add the lemon juice and serve.