In a large saucepan, bring lentils, turmeric, chili, and water to a boil. Reduce heat so mixture bubbles steadily and cook uncovered for 25 minutes.
Mix in tamarind liquid and an additional ½ cup of water. Let bubble for an additional minute. Remove from heat and puree the dal with an immersion blender, food processor, or blender for 1 minute to make dal saucy but not soupy.
Make spice mix by heating oil in a small pan over high heat. Add mustard seeds, cover, and let sputter for 30 seconds. When popping begins to subside, add garlic and cook, uncovered, for 15 seconds to lightly brown. Add curry leaves, if using, cover and cook another 10-15 seconds.
Add spice mix into saucepan with dal and stir to combine. Transfer to a serving bowl, sprinkle top with cilantro, and serve hot.