Add the flank steak to a large baking dish and season it with salt and pepper. In a bowl, whisk together the olive oil, brown sugar, and minced garlic, then cover the steak with this mixture and marinate in the fridge for anywhere from 2 hours to overnight.

When ready to cook, preheat the broiler in your oven, moving the oven rack as close to the broiler as possible. Place the marinated steak on a broiler pan or baking sheet and broil on each side for about 5 minutes for medium to medium-well doneness.

Allow the steak to rest for 10 minutes before slicing thinly against the grain. Serve it with the chimichurri sauce.

To make chimichurri, combine parsley, cilantro, oregano, and minced garlic in a food processor and pulse until finely chopped. Add red wine vinegar and pulse again. With the processor running, stream in olive oil until combined. Season with salt, pepper, and crushed red pepper flakes to taste. Store in the fridge for up to a week.