Marinate the sardines for a couple of hours, turning them gently from time to time in their marinade. Wrap each in 1 or 2 sorrel leaves, placing it in a large gratin dish, the bottom of which has been scattered with the chopped onions—the dish should be just large enough to hold them pressed without forcing side by side. Dribble over the marinade, adding a bit more white wine if the bottom of the baking dish is not completely moistened. Press a sheet of aluminum foil over the surface and cook in a 450° oven for some 12 minutes—or a bit longer—until the liquid has come to a bubbling boil and the sorrel has turned grayish, clinging, film-like, to the form of each sardine. Leave for another 10 minutes or so in a warm but not hot oven (remove the dish, turn off the oven, cool it for a few minutes, and return the dish to the oven, still covered with the foil) to steep, the liquid evaporating a bit more—only a spoonful or so of the cooking juices should remain for each guest.