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

When ready to cook the steak, grill, broil, or pan sear it as desired. Preheat the broiler and adjust the oven rack to the highest position. Put the steak on a broiler pan or baking sheet and broil each side for about 5 minutes, achieving medium to medium-well doneness.

Let the steak rest for 10 minutes before slicing it thinly against the grain. Serve immediately with the chimichurri.

To prepare the chimichurri, combine parsley, cilantro, oregano, and garlic in a food processor and pulse until small pieces remain. Add the vinegar and pulse again. While processing, stream in the olive oil and mix until just combined. Stir in salt, pepper, and red pepper flakes, adjusting seasoning to taste. Store in the fridge sealed for up to a week.