Prepare grill. In a very small saucepan heat rosemary and vinegar over low heat until hot (do not let boil). Remove pan from heat and let mixture stand, covered, 20 minutes.
In a metal measure heat oil over low heat until warm. (Heating thins oil, making it easier to brush sparingly over onions.) Arrange onion slices in one layer on trays, keeping slices intact, and brush both sides of each slice lightly with oil. Season onions with salt and pepper and grill in batches on a lightly oiled rack set 5 to 6 inches over glowing coals 4 to 6 minutes on each side, or until lightly charred and softened. Transfer onions as grilled to a large bowl, separating rings, and toss with vinegar mixture, parsley, and salt and pepper to taste.
Serve onions warm or at room temperature.
Recipe courtesy of Gourmet Magazine