1 whole flank steak 2 to 2-1/2 lbs. trimmed of excess fat
Kosher salt and freshly ground black pepper
1 cup pesto
¾ cup toasted pine nuts
3 ounces thinly sliced prosciutto
4 ounces thinly sliced low-moisture mozzarella cheese
Extra-virgin olive oil
Lay steak on a cutting board with grain running parallel to the edge of the counter. Trim left and right edge to form a clean rectangle and reserve scraps for another use. Hold steak flat with your non-knife hand and, with a sharp boning knife, carefully butterfly the steak, leaving the back edge attached by 1/2- to ¼-inch of meat. Open up steak and flatten the seam gently with your hand to form a large perfect rectangle.
Season steak on exposed side with salt and pepper. Spread half of pesto evenly over steak, leaving a 1-inch boarder at the top and the bottom. Sprinkle with pine nuts. Layer prosciutto and mozzarella over steak, leaving the border at the top and bottom.
Carefully roll the steak away from you (the grain should run width-wise), tightening as you go until it is rolled into a cylinder. Let it rest with its seam-side down. Tie the beef tightly with twine, spacing the ties evenly every 1-1/2 inches. Insert a skewer through each piece of twine. Using a sharp knife, carefully cut between the ties to make the pinwheels. Season with salt and pepper.
Place the pinwheels on the hot side of the grill and cook without moving until well charred on first side, about 3 minutes. Flip steaks and char second side, about 3 minutes longer. Transfer to cooler side of grill, cover, and cook until an instant read thermometer inserted into the center registers 120 degrees F for medium-rare or 130 degrees F for medium. Transfer to a platter, let rest for 5 minutes and serve with remaining pesto and a drizzle of olive oil.