Grilled Sweet-Potato Salad

In a large saucepan cover potatoes with salted cold water by 1 inch and simmer, covered, until just tender, about 15 to 30 minutes, depending on size of potatoes. In a colander drain potatoes and rinse under cold water to cool. With a sharp knife peel potatoes and cut crosswise into 1/2-inch-thick slices. While potatoes are boiling, prepare grill. In a small bowl whisk together oil, salt, and cumin and brush some onto both sides of potato slices, reserving remaining cumin oil. Grill slices on an oiled rack set 5 to 6 inches over glowing coals until golden brown, about 1 minute on each side, and transfer with tongs to a platter. Thinly slice scallion diagonally and sprinkle over potatoes. Whisk lime juice into remaining cumin oil with salt and pepper to taste and drizzle over potatoes. Serve potato salad with lime wedges and sprinkled with coriander.