Heat a clean griddle and brush with 1/2 of the oil. Brush onion slices lightly with remaining oil on both sides, season with salt and pepper and place on griddle. Grill for 3 to 5 minutes on each side, or until softened and lightly charred. Transfer onions to a bowl, drizzle with vinegar and sprinkle with rosemary. Toss to separate rings and season with more salt and pepper as needed.
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