Add Toor dal, turmeric and water to a medium sized heavy bottomed vessel like a crockpot and cook on medium flame, stirring often till the dal cooks and becomes mushy.
Pressure cook the dal, turmeric and water for 3 whistles and wait for natural pressure release.
In the pot, add the Toor Dal, turmeric and 3 Cups of water. Close the lid with the vent in sealing position. Set to manual or pressure cook mode and cook at high for 5 minutes. When you hear the beep, release pressure naturally for 5 minutes, and then manual pressure release for another five. Now open the lid and switch to sauté mode for the next step.
In a small bowl, grate the tomatillo and garlic together. Finely chop the Thai peppers.
Add the tomatillo with garlic, green chili, and salt to the dal. Let it cook for 5 minutes till the mixture thickens and becomes cohesive (you can do this on medium size pan over stove top or switching to saute mode in the Instant Pot). Taste and adjust for salt according to your taste. Transfer to a serving bowl.
Add a couple of spoonfuls of oil to a small pot or pan and heat over medium heat until the oil is hot. Add mustard seeds. The pan will sizzle and the mustard seeds will sputter and pop. Turn off the flame and pour over the top of the dal. Serve.