Seasoned Lentil Stew

Bring dal, 1 1/2 cups water, and turmeric to a boil in a small saucepan. Reduce heat to very low and cook, covered (check occasionally, and if mixture appears dried out, add up to about 1/4 cup water), until peas are soft and break when pressed with a spoon, about 30 minutes (45 minutes if using split peas). Meanwhile, dissolve tamarind concentrate in 2 tablespoons hot water, breaking it up with your fingers. Heat oil in a 4-quart heavy saucepan over medium-high heat until it shimmers, then cook fenugreek seeds and asafetida, stirring, until fenugreek seeds begin to brown, about 30 seconds. Stir in remaining 2 cups water, potato, vegetables, chile, cilantro, sambar powder, tamarind mixture, and 1 1/2 teaspoons salt. Bring to a boil, then simmer, uncovered, until all vegetables are tender, about 20 minutes. Mash dal in saucepan with a potato masher or back of a spoon to form a coarse paste, then stir into vegetable mixture. Simmer 10 minutes, stirring in water as necessary to thin (consistency should be thinner than pea soup). Season with salt and lemon juice.