Add the flank steak to a large baking dish and season it with the salt and pepper. In a bowl, whisk together the olive oil, brown sugar and garlic cloves, then cover the steak with it and place it in the fridge to marinate. I marinate anywhere from 2 hours to overnight.
When you’re ready to cook the steak, you can grill, broil or pan sear it to your liking. I tend to broil it as I find that easiest. Preheat the broiler in your oven and move the oven rack as close as possible. Place the steak on a broiler pan or baking sheet and broil on each side for about 5 minutes. This will result to a medium to medium-well doneness in my oven.
Allow the steak to rest for 10 minutes before slicing it thinly against the grain. Serve it immediately with the chimichurri.
Combine the parsley, cilantro, oregano and garlic in a food processor and pulse until small leaves and pieces remain. Add in the vinegar and pulse once more. With the processor going, stream in the olive oil and mix until just combined. Stir in the salt, pepper and red pepper flakes. Taste and season additionally if needed. Store sealed in the fridge for up to a week or so.