Bring lentils and 3 cups water to a boil with turmeric in a 2-quart heavy saucepan, then gently simmer, partially covered, stirring occasionally, until falling apart, about 20 minutes.
When lentils are cooked, heat oil in a small heavy skillet over medium-high heat until it shimmers, then cook mustard seeds, cumin seeds, and red pepper flakes until mustard seeds begin to pop and/or turn gray and cumin seeds brown, about 1 minute. Stir spice mixture into lentils with coconut milk, cilantro, lemon juice, and salt and bring to a simmer.
6 servings