Not too much prep for this recipe. Chop the parsley, oregano and garlic. Get everything ready for the marinade and put it all together.
You can make this in a bowl and add the meat to it but I made it in a bowl and marinated the meat in a large freezer zip lock bag. Either way, add the olive oil, vinegar, sherry, cilantro, oregano, lemon juice, garlic, and some salt in pepper into a bowl and stir it together.
Remove about ⅓ of the mixture and reserve for adding at the end.
Add the flank steak and the remaining mixture to a zip lock bag and refrigerate for several hours. About 3 hours would be great but 2 will do. Be sure to remove the steak from the refrigerator 20 -30 minutes before you start grilling. It's always better to grill meat at room temperature. Grill the meat directly over the heat. Depending on your grill and how you like your meat, cook for about 5 minutes per side turning the meat over only once. If the meat looks like it is cooking too fast and is starting to burn, just move it over to a cooler spot on the grill. I brushed a little of the marinade on the steak before turning over.
Remove the steak from the grill, slice it across the grain into 2 inch wide pieces, plate it, and top it with the reserved Chimichurri sauce.