In a large pot, add lentils. Cover with water by 2 inches. Bring to a boil over high heat. Reduce heat to a simmer. Cook until tender, about 20 minutes. Drain.
Preparation
Rinse, peel, and dice mango. Rinse and dice bell pepper.
Rinse lemon and cut in half. In a small bowl, squeeze juice. Discard seeds.
If using, rinse and chop cilantro. Chop walnuts or toasted coconut.
In a large bowl, use a fork to whisk together lemon juice, oil, cumin, salt, and pepper. Add cooked lentils, mango, and bell pepper. Mix well. If using, stir in herbs and nuts or coconut.