This dish is traditionally prepared with sambar spice, a mixture of ground spices and lentils—what we have here is a simplified version that still provides lots of layers of flavor and just the right amount of heat. This recipe has a lot of steps, so give yourself plenty of time for the pressure cooker to return to pressure between cooking the lentils and the sweet potatoes. Despite the high involvement, the different elements of this recipe all come together at the same time.
How to Make It
Select SAUTÉ setting on a 6-quart programmable pressure multicooker (such as Instant Pot). Select HIGH temperature setting, and allow to preheat. Add 2 tablespoons of the ghee, and cook until melted and fragrant. Add garlic, cayenne, tumeric, curry powder, 1 teaspoon of the mustard seeds, and 1 teaspoon of the cumin seeds; cook, stirring often, until mixture is fragrant and garlic is just starting to brown, about 2 minutes. Stir in tomatoes, lentils, salt, and 2 1/2 cups of the water. Cover cooker with lid, and lock in place. Turn steam release handle to SEALING position.
Select MANUAL setting on HIGH pressure for 10 minutes. (It will take 5 to 15 minutes for cooker to come up to pressure before cooking begins.)
Let pressure release naturally for 10 minutes. Carefully turn steam release handle to VENTING position, and let steam fully escape (float valve will drop). (This will take 5 to 15 minutes.) Remove lid from cooker, and turn off cooker. Transfer 1 1/2 cups of the lentil mixture to a blender. Remove center piece of blender lid (to allow steam to escape); secure lid on blender, and place a clean towel over opening in lid. Process until smooth, about 10 seconds. Transfer to a large bowl along with remaining lentil mixture. Wipe cooker insert clean.
Add sweet potato, onions, tamarind, and remaining 3 cups water to cooker. Cover cooker with lid, and lock in place. Turn steam release handle to SEALING position.
Select MANUAL setting on HIGH pressure for 4 minutes. (It will take 5 to 15 minutes for cooker to come up to pressure before cooking begins.)
While vegetables cook, add asafetida and remaining 1 teaspoon each mustard seeds and cumin seeds to a spice grinder or coffee grinder; process until finely ground, about 15 seconds.
Let pressure in cooker release naturally for 10 minutes. Carefully turn steam release handle to VENTING position, and let steam fully escape (float valve will drop). (This will take 5 to 15 minutes.) Remove lid from cooker, and turn off cooker. Stir sweet potato mixture into lentil mixture.
Cook ground spice mixture and remaining 2 tablespoons ghee in a small skillet over low, stirring often, until spice mixture is fragrant and darkens, 3 to 4 minutes. Stir into sweet potato-lentil mixture.
Divide soup evenly among 4 bowls. Garnish with basil leaves, and, if desired, sprinkle with fried onions.