Pat steak dry with paper towels and season with salt & pepper. Brush onion rounds with 1 tbsp oil and season with salt. Grill steak over hot fire until well browned and cooked to desired doneness, about 5 mins per side for medium rare. Grill onion until charred and soft, about 5 mins per side. Transfer steak and onion to cutting board and tent with foil.
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When cool enough to handle, chop onion coarsely. Heat remaining oil in small saucepan over medium heat until shimmering. Add onion, vinegar, sugar, cayenne, and any accumulated beef juices and simmer until thickened, 3-5 mins. Off heat, stir in parsley. Slice steak thinly against grain. Serve steak with relish.
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