Provolone and Arugula-Stuffed Flank Steak

Heat grill to medium-high. Butterfly the steak: Cut it in half horizontally (don't cut all the way through) and open it like a book. Rub the garlic over the inside of the steak. Scatter 1 heaping cup of arugula over the steak, then lay the cheese on top. Roll up the steak and tie with kitchen twine. Season with 1/2 teaspoon each salt and pepper. Grill the steak, covered, turning occasionally and brushing with the vinegar, 15 to 18 minutes for medium-rare. Let rest for at least 5 minutes before slicing. In a large bowl, toss the tomatoes and onion with the lemon juice, oil, and 1/4 teaspoon each salt and pepper. Toss with the remaining arugula; serve with the steak.