Special equipment: a meat mallet; kitchen twine
For the flank steak: Prepare an outdoor grill for cooking over direct and indirect heat (if your grill has a thermometer, aim to keep the temperature at about 350 degrees F throughout grilling).
Heat a medium skillet over medium heat. When the skillet is hot, add 1 tablespoon olive oil. Add the chorizo and cook, stirring and finely crumbling with a wooden spoon until cooked through, about 4 minutes. Drain on paper towels and let cool.
Set the flank steak on a cutting board. Use a knife to score the steak down the middle against the grain, halfway through its thickness. Working from the center line, out toward the edges, cut thin flaps from the score to the edge, like a gatefold. Pound with the flat side of a meat mallet to an even thickness of about 1/2 inch. Sprinkle both sides of the steak with salt and pepper and brush with the remaining 2 tablespoons olive oil.
Chop the cilantro, parsley, olives, pickled jalapenos and garlic together on a cutting board to make a chunky paste. Brush the steak with the mustard. Spread the herb paste over the steak, leaving empty a 1-inch border around the edges. Pat to adhere the paste to the meat. Starting a third of the way in from the right, make 4 equidistant lines of egg. Between the egg lines, mound lines of chorizo, then carrots and then peppers, covering all but that first third of the steak's surface. Roll the steak around the fillings like a jelly roll, from the filled to the empty side. Tie closed at 2-inch intervals around the circumference with kitchen twine. Tie 1 more piece of twine lengthwise around the meat.
Lightly oil the grill grates. Grill the steak on the direct-heat side of the grill to brown all 4 sides, about 2 minutes per side. Move the steak to the indirect-heat side of the grill, cover and cook, turning occasionally and testing the temperature frequently, until the very center reads 115 degrees F on an instant-read thermometer, for a steak that is well-done on the outside and medium-rare in the center, 25 to 30 minutes more. Transfer the matambre to a cutting board to rest.
For the chimichurri sauce: Finely chop the cilantro, parsley and garlic on a cutting board. Transfer to a small bowl and stir in the red pepper flakes, vinegar and 1/2 teaspoon salt. Stir in the olive oil.
Untie and slice the flank steak into 6-thick slices on a slight bias. Serve with the chimichurri sauce for drizzling.
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