Combine toor dal, 1⁄2 teaspoon turmeric, 1 teaspoon salt and 3 cups water in a pot and bring to a simmer. Cook until the lentils are mushy, about 30 minutes. Use an immersion blender to puree the lentils to a thick paste and set aside. If you don’t have a blender, you can mash them with a wooden spoon.
For the vegetables
Once the lentils start to simmer, begin cooking the vegetables in a second pot. Add 2 teaspoons of oil to a saucepan along with the pearl onions. Saute for about 2 minutes, then add the remaining vegetables.
Saute the vegetables for another 2 minutes, then add red chili powder. Stir together.
Pour 3 Cups of water into the vegetables. Bring to a simmer over medium heat and cook until vegetables are just cooked, about 20 minutes.
Add 1⁄4 teaspoon turmeric, sambar powder and 3⁄4 teaspoon salt to the cooking vegetables. Cook for an additional 2-3 minutes until the vegetable drumsticks are softened.
Add the tamarind paste and sugar to the vegetables.
Then add the mashed lentils to the vegetables and stir together. Simmer on medium low until it comes to a boil. Once the sambar has a frothy layer on top, turn off the heat.
Stir in the coriander leaves. Cover and set aside.
For the tempering
In a small pan, heat 1 tablespoon of oil over medium heat. Add the mustard seeds, fenugreek leaves and dried chili peppers. Cook until the mustard seeds start to pop, about 5 minutes.
Add curry leaves and continue cooking. Once the curry leaves are crispy, stir in the hing.
Pour the tempering into the hot sambar and stir in. Cover and let it sit for about 5 minutes to impart the flavor of the tempering to the soup.
Notes
To store:Leftover sambar can be stored in an airtight container in the refrigerator for up to 3 days. Reheat it in a saucepan set over low heat or in the microwave until heated through.To freeze: Place the cooled sambar in an airtight, freezer-safe container and it will keep for up to 3 months. Thaw completely before reheating according to the instructions above.