Add Toor dal 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 and water for 3 whistles and wait for natural pressure release.
In the pot, add the Toor Dal 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 switch to sauté mode
Grate the tomatillo and garlic. Finely chop the Thai peppers. On low flame or sauté (Instant Pot), add the tomatillo, green chili, and salt to the dal. Let it cook for 5 minutes till the mixture thickens and becomes cohesive. Taste and adjust for salt according to your taste.
Pour it in a serving bowl.
Heat 2 Tablespoons of oil in a small pan, when it gets hot to touch, add the mustard seeds. The mustard seeds will sputter. Turn off the heat and top the Tomatillo Dal immediately (the tempering will slightly splatter, so be careful). Enjoy with rice.