In a large, heavy skillet over low heat, warm the olive oil. Add the onions, herbes de Provence, sugar and salt. Cover and cook, stirring occasionally, until the onions are golden and juicy, about 30 minutes. Uncover the skillet and cook, stirring occasionally, until the onion juices have evaporated, 15 to 20 minutes. Raise the heat slightly and cook, stirring often, until the onions are lightly and evenly browned, another 10 to 15 minutes. Remove from the heat and cool slightly.