Using the Sauté function on High, heat 3 teaspoons oil in the inner pot for about 1 minute, until shimmering. Add the onion, ginger, garlic, curry powder, 10 curry leaves and salt; cook for about 1 minute, until fragrant.
Stir in the lentils and water. Secure the lid and cook on high pressure for 10 minutes.
Once the cooking is complete, let the pressure release naturally for 10 minutes, then quick-release the remaining pressure. Let the soup cool slightly.
Working in batches, transfer half of the soup to a blender and blend on high speed until smooth (you can also use an immersion blender to purée the soup.) Return the pureed soup to the pot, stir in the lime juice, and season with additional salt if needed. Swirl in the yogurt (if using).
Heat the remaining oil in a small skillet over medium heat for about 1 minute. Add the remaining 5 curry leaves, red chile pieces and yellow mustard seeds; cook for about 1 minute, until fragrant and the seeds begin to pop. Pour the tempered mixture over the soup and serve.