Season the steak liberally with salt and pepper.

In a mixing bowl, whisk together the balsamic vinegar, brown sugar, garlic, rosemary and chili flakes. Add about 1/3 of a cup of olive oil. Place the steak in a resealable plastic bag and pour the marinade into it. Marinate for a minimum of 2 hours and preferably overnight in the fridge.

Remove the steak from the fridge 30 minutes prior to use. Heat a grill over medium-high heat. Wipe off the bits of the marinade from the steak and drizzle it with olive oil.

Place the steak on the grill and cook 2 to 4 minutes per side, or until the meat has grill marks. Let the steak rest for 10 minutes, then slice across the grain and serve.