Crush the rosemary needles lightly with a rolling pin to release their volatile oil. In a small bowl, combine the rosemary with ¼ cup of the olive oil, the garlic, cracked peppercorns, and 2 tablespoons of the balsamic vinegar.

Place the flank steak in a shallow non-reactive dish and pour the marinade over. Turn the steak to coat both sides, pressing the spices onto the steak. Marinate for 30 minutes.

Preheat the grill to medium heat. Clean the grill grates with a stiff brush, then oil liberally to prevent sticking. (Note: To use a grill pan, preheat the pan to medium-high heat; add a very light coating of oil and heat through.) Remove the steak from the marinade. Brush off any garlic or spices clinging to the steak and season both sides well with salt and pepper. Place the steak on the prepared grill or grill pan and cook 2 to 3 minutes, then, without turning the steak over, rotate it 90 degrees and place back on the grill or grill pan; this will produce beautiful brown cross-hatch grill marks. Cook for an additional 2 to 3 minutes.

Brush the top of the steak generously with balsamic vinegar. Turn the steak over and brush the other side generously with balsamic vinegar. Cook 2 to 3 minutes, then rotate the steak 90 degrees; cook to an internal temperature of 125°F for medium rare, about 2 to 3 minutes or to 135°F for medium, approximately 4 to 5 minutes. Transfer to a platter and tent loosely with foil.

Clean and re-oil the grill grates or grill pan. Brush the onion slices on both sides with olive oil; season well with salt and pepper. Grill the onions approximately 2 minutes on each side. Brush the onions with the remaining balsamic vinegar and cook for an additional 1 to 2 minutes. Slice the flank steak on the bias into thin slices, cutting across the grain. Arrange the slices on a serving platter; pour over any juices that collected on the platter where the steak was held. Place the grilled balsamic onions on the platter with the steak. Garnish with fresh rosemary sprigs and serve immediately.