Put the lentils in a fine strainer and pick through them, discarding any bits of stone that may be present. Rinse under cold running water. Place in a pot and cover with 3 to 4 inches of water, bring to a boil, then reduce to a simmer.
Check lentils after 15 to 20 minutes; they should have slight resistance to the tooth—you want them al dente. Remove from heat, drain, and place under cold running water to stop the cooking process. Once cooled slightly, combine the lentils with mango and onion in a large bowl.
In a small jar or bowl, combine the olive oil, coriander, cumin, lime juice, and cilantro leaves. Pour over the lentils, mango, and onion. Toss to combine. Garnish with some more cilantro leaves.