In a small bowl, whisk together the lime zest and juice and ¼ teaspoon salt. Stir in the onion and chilies; let stand for about 15 minutes.
Meanwhile, in a large microwave-safe bowl, combine the sweet potatoes and 3 tablespoons water. Cover and microwave on high until a skewer inserted into the largest piece meets no resistance, about 10 minutes, stirring once halfway through. Immediately drain the sweet potatoes in a colander and return the potatoes to the bowl.
To the warm potatoes, add the onion-chili mixture and any liquid, the coconut milk and cilantro; toss gently. Taste and season with salt and pepper. Serve at room temperature or cover and refrigerate until chilled, about 1 hour. Sprinkle with additional cilantro just before serving.
Optional garnish: Crushed corn nuts OR chopped roasted peanuts
