Cut flank steak into 8-ounce portions and lightly tenderize with mallet. Dust with crushed black pepper and kosher salt. Press salt and pepper into steak with the palm of hand. Transfer steaks to a baking dish and place in the refrigerator. Let marinate for 4 to 8 hours.
In a medium sauce pan over medium heat, add extra-virgin olive oil and onions and lightly saute until onions are caramelized. Add garlic and cook until garlic begins to turn brown. Deglaze with wine and balsamic vinegar. Add sugar and let simmer and reduce for 20 to 30 minutes until mixture reduces to 3/4 cup.
Place steaks over a hot grill. Move once on first side to mark the steaks, cook for 5 minutes and flip, again only moving once to mark steak, cook for 4 minutes. When desired doneness is achieved, remove steaks from grill and let rest for 3 to 4 minutes.
Strain wine mixture and return to heat to reduce for 5 more minutes or until thick syrup is created. Serve with the steaks.