Mix the lentils and turmeric, add water and cook until tender in a pressure cooker. Or cover the peas with an inch of water in a pot and bring to a boil, then slap on a lid, lower to simmer, and boil for 30-45 minutes or until the peas are tender enough to mash. Add more water if needed as they cook.
Prepare the ground masala by heating 1 tsp of oil. Add to it the coriander seeds, chana dal, fenugreek seeds and chillies and saute until the coriander seeds and dal turn a few shades darker and are lightly golden-brown. Remove to a blender, add enough water to keep the blades moving, and process to a smooth paste.
Heat 1 tsp of the oil, add the pearl onions and stir-fry until the start to turn golden. Add the tamarind extract and let the mixture cook about 8 minutes.
Add the ground masala and stir well to mix. Add the cooked tuvar dal and bring everything to a boil.
Lower the heat to a simmer, add salt to taste, and let the sambar cook for 10 minutes so all the flavors have a chance to meld together.
To temper the sambar-- an important flavor-building step-- heat the last teaspoon of oil in a small saucepan and add the mustard seeds. When they crackle, add the asafetida and curry leaves. Fry for a few seconds, turn off the heat, and pour over the sambar. Mix well.