While dhal cooks, dissolve tamarind concentrate in 1 tablespoon hot water and set aside.
In a 3-quart covered saucepan over medium high heat, heat mustard seeds, fenugreek seeds, and asafetida in oil until the mustard seeds begin to pop.
After the seeds have popped for a few moments, add 2 cups water, salt, spice blend, dissolved tamarind, potato, tomatoes, onion, green beans, green chile, cilantro and curry leaves and simmer over medium low heat until the potatoes are tender, about 20 minutes.
When dhal is cooked and completely tender, mash with a potato masher or back of a spoon 6 to 8 times, to break up roughly. Stir into cooked vegetable and spice mixture in saucepan and simmer for another 10 minutes. Add more water if necessary to attain a soupy consistency.