Combine vinegar, sugar, rosemary and 1 1/2 tsp. salt in a ziplock bag. Add steak to bag, seal and turn bag several times to coat meat with marinade. Refrigerate 20 minutes, turning once or twice to redistribute marinade.
Preheat a gas grill to high. Remove steak from marinade and sprinkle both sides with salt and pepper. Discard leftover marinade. Grill steak for 4 to 6 minutes, carefully turn over, and continue to grill until meat reaches desired doneness, 3 to 4 minutes more for medium-rare, depending on thickness of meat and heat of grill. (Alternatively, grill steak on a lightly oiled ridged grill pan over medium-high heat on stove, for same amount of time.)
Transfer steak to a cutting board, tent loosely with foil and let stand 5 minutes. Slice steak thinly against grain and serve.
I doubled this recipe for a large 2.5 lb. flank steak and it came out great! It was incredibly easy and all ingredients that I always have available. We cooked it on our green egg and it smelled so good that the neighbors were curious what it was. They ended up coming over to share in our meal. The balsamic vinegar and sugar makes for a great caramelized crunch on the steak and tenderizes it as well. I also used the basis of the marinade recipe and boiled it down to make a balsamic reduction glaze for chicken that I served with the meal too. Great both ways! 5 stars for being easy, few recipes, delicious, family and neighborhood pleasing.
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