Bring lentils and 3 cups water to a boil in a heavy-bottomed pan. Cook, partially covered, until very tender, 40 to 45 minutes. Stir in salt and cilantro; remove from heat. Add 8 lemon slices; stir to combine. Transfer to a serving bowl; set aside.
Heat oil in a small saute pan over medium-high heat; add mustard seeds. After a few seconds, seeds should begin to pop; add chiles. When chiles have darkened, add garlic; stir to combine. Top lentils with mustard seed mixture.
