Cook the black lentils in salted water over low to medium heat until very tender, 15 to 30 minutes. Drain the lentils and reserve.
In a skillet over medium to high heat, fry the mustard seeds in the vegetable oil until they begin to pop, 10 to 30 seconds. Add the red pepper flakes and mint, and continue cooking until the mint leaves wilt slightly, 10 to 30 seco