Preheat the oven to 425°. Using a sharp knife, carefully peel the limes; be sure to remove all of the bitter white pith. Working over a bowl, cut in between the membranes to release the lime sections into the bowl. Cut each section crosswise into quarters. Return the limes to the bowl and stir in the onion, cilantro and jalapeño. Add 1 tablespoon of the oil, the sugar and a pinch of salt and toss well.