In a large bowl, combine the potatoes, onions, mangos, and bell peppers. Add the oil, lime juice, cinnamon, cumin, nutmeg, salt, and cayenne, and toss well to coat evenly.
Lay 2 large pieces of heavy aluminum foil in a stack on a work surface. Mound the sweet potato mixture into the middle and wrap in the foil, turning up the edges to make a tight, well-sealed package. Place directly on the grill and cook until the vegetables are tender, about 30 minutes.
(Alternately, preheat the oven to 450 degrees F. Place the potato-mango package in the middle of the oven and roast until the potatoes begin to soften, 45 to 50 minutes. Uncover and roast until tender and starting to caramelize, 15 to 20 minutes.)
Remove from the grill and let cool slightly before unwrapping. Transfer the sweet potato mixture to a large, decorative bowl and adjust the seasoning, to taste. Sprinkle with peanuts and fresh cilantro and serve.