To make the spice mixture, heat a small frying pan or skillet. Then, add the coconut, sesame, coriander, and cumin seeds. Toast the seeds for 1 minute, stirring constantly.
Immediately transfer the spices to a spice grinder or small blender. Grind the spices into a coarse powder and set aside.
Heat a medium pot to medium-high heat. Add the red onion. Saute for 2-3 minutes until the onion starts to soften.
Stir in the garlic, the coconut spice mixture, turmeric, red chili flakes, nutmeg, and salt. Mix the spices into the vegetables.
Add the red lentils, mango cubes, and vegetable broth. Bring the pot to a simmer. Simmer uncovered until the lentils are tender. Stir occasionally and use a wooden spoon to mash some of the mangoes. Add more vegetable broth or water if needed. The longer you simmer, the creamier the dal gets.
Once the lentils are tender, stir in the chopped cilantro and lime juice.
Serve hot with chopped cilantro, lime wedges, or crushed red chili if desired.