About 45 minutes before serving, light a charcoal fire and let it burn down to medium-hot. Carefully trim the root ends off the onion, trim 1/2-inch from the top, and cut away any withered outer leaves. Position the grill 4 to 6 inches above the coals and lay a double folded square of aluminum foil on one side of the grill. Brush the onions lightly with the oil and season to taste. Lay the white part of the onion on the grill over the fire and the green part on the foil. Lay a heavy cast iron skillet over them to press the onions down onto the grill, so they will have nice grill marks. Grill them, turning occasionally, for 7 to 13 minutes, depending on the size of the onions. They are done when the white bulb is browned and well marked, and the green tops are soft. Remove to a warm serving platter and squeeze a little lime juice over the top. Serve warm, accompanied by the remaining lime wedges.

Note: This dish works best with knob onions (overgrown green onions with a bulb about 1-inch across). If these are not available, use the largest green onions you can find so that they don't soften too quickly when grilling.