Look at the steak and determine the direction of the fibers or the grain. This may be easier before marinating. This will be important when you’re ready to slice.
2. In a bowl, stir together the Porter, syrup, Dijon mustard, Worcestershire sauce, garlic, salt, and pepper. Place the steak in a shallow dish; pour marinade over steak. Cover and refrigerate overnight. Flip the steak several times.
Remove steak from marinade. Heat a cast iron skillet over medium-high heat. When hot, add the oil. Cook 3-5 minutes per side depending on the thickness of the steak. Flank steaks are best served rare to medium rare but, if desired, cook longer. Let rest 5-10 minutes.
Slice the steak in thin slices diagonally against the grain. Slicing with the grain with result in a fibrous steak that is tough to chew.
Make the Chimichurri by placing all the ingredients in a blender. Blend until smooth. Season to taste with salt and pepper.