Add the parsley, oregano, garlic in a food processor and process until finely pureed. When necessary, scrape down the sides of the bowl to get everything processed evenly. (See Note)

Add the herbs to a small bowl, then whisk together the herbs with the olive oil, vinegar, salt and red pepper flakes. Taste and adjust the seasoning.

Use right away or store, tightly sealed, in the refrigerator for a couple of days.

Place the flank steak on a cutting board in front of you with the short end facing you, and the meat fibers running perpendicular to you.

Butterfly the flank steak. Using a long, thin, and sharp knife, like a boning knife, cut the flank steak in half through the thickness of the meat. Beginning at the outer long side, either your right or left. Cut through the middle thickness of the steak, pulling back the top layer as you go. Keep the knife blade level to the countertop so you do not cut up or down, just straight across. Cut the flank steak open until you reach a half an inch from cutting all the way through at the opposite side. Open the steak like a book.

Sprinkle about a 1/4 teaspoon of Kosher salt over both sides of the flank steak.

Spread the chimichurri sauce over the side of the flank steak you just cut open.

Staring at the long side, either right or left depending on your dominant hand, roll up the steak with the meat grain running long and perpendicular to you.

Once rolled up, tie up your flank steak roll with kitchen twine. I use 5 ties up the length of the rolled steak. Trim off the long ends of the string.

Rub any escaped chimichurri sauce over the exterior of the steak. Cook right away or let marinate wrapped in plastic in the refrigerator. Marinate for no more than 8 hours.

Prepare the grill for direct and indirect heat areas. Oil the grill before placing the meat down. When the grill is nice and hot place the rolled flank steak directly over the flames on the grill (direct heat). Sear the meat. Turn the meat every couple of minutes and sear all sides of the surface, about 8 to 10 minutes total. Once the meat is seared, move it over to the side of the grill prepared for indirect heat. Cook for about 10 minutes, covered. Check the internal temperature. If the temperature in the middle of the meat is around 120˚F - 125˚F (49˚C- 52˚C), the steak is done cooking.

Remove the flank steak from the grill and let it rest covered with foil for 10 minutes. This resting period should produce medium-rare rolled flank steak, about 130˚F - 135˚F (54˚C- 57˚C).

Preheat the oven to 375˚F (190˚C / Gas Mark 5). Heat and lightly oil a skillet or grill pan large enough to hold your rolled flank steak. When your pan is almost smoking, sear your flank steak, turning it over every 2 minutes searing the steak all over. When the flank steak is seared, place the pan with the flank steak in the oven and cook. After 10 minutes, check the internal temperature. Stop cooking when the internal temperature in the middle of the rolled steak reaches between 120˚F - 125˚F (49˚C- 52˚C). Place the rolled flank steak on a cutting board and let it rest covered in foil for 10 minutes. Medium-rare meat has an internal temperature of, 130˚F - 135˚F (54˚C- 57˚C).

Leftovers make great sandwiches with a chimichurri mayo, lettuce, tomato, and your favorite bread or roll.