1Make the chimichurri: roughly chop parsley, oregano, and garlic and add to a small bowl. Pour in vinegar and olive oil, sprinkle in red pepper flakes, salt, and pepper. Stir well and set aside while you grill.
2Pat the flank steak dry with paper towels and season both sides generously with salt and pepper.
3Heat your grill to high—you want it hot enough that you can hold your hand 2 inches above the grates for only 2 seconds.
4Oil the grill grates lightly to prevent sticking.
5Place the steak on the grill and let it sit undisturbed for 4 to 5 minutes to develop a crust. Flip once and grill for another 3 to 4 minutes for medium-rare (the internal temp should hit 130°F).
6Transfer the steak to a cutting board and rest for 5 minutes—this keeps it juicy.
7Slice the steak thinly against the grain. Spoon chimichurri over each plate or serve on the side for dipping.