Preheat the oven to 400°F. Spray a rimmed baking sheet with nonstick spray, or line with parchment.
In a large saucepan, cover the yams with plenty of water and bring to a boil. Cook for 10 minutes, then drain. Cut the yams into rounds, about ½ inch thick.
On the prepared baking sheet, arrange the sliced yams and red pepper strips in a single layer. Bake for 12 to 15 minutes, until the yams are easily pierced with a fork.
Meanwhile, in a frying pan, heat the oil over medium heat for 1 minute. Add the mustard seeds, cayenne, cinnamon and cumin; stir fry for 2 minutes, or until the spices begin to pop. Add the pecans and stir fry for 2 more minutes until the nuts have browned a little on both sides (don't burn them). Remove from heat and reserve the spices in the pan.
Remove the yams and red peppers from the oven. Using a spatula, carefully transfer them onto a serving plate, making a single layer.
Drizzle the lime juice and sesame oil over the yams and sprinkle with salt. Scatter slivers of red onion on top. Using a spoon and a rubber spatula, scrape the contents of the frying pan (pecans, oil and spices) evenly over the yams. Let the salad rest for 10 to 15 minutes, then garnish with the chopped cilantro and serve.