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1
Select SAUTE 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.
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2
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.)
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3
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.
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4
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.
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5
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.)
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6
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.
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7
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.
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8
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.
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9
Divide soup evenly among 4 bowls. Garnish with basil leaves, and, if desired, sprinkle with fried onions.