Before you make the chimichurri steak, you need to make the sauce which will used for marination.
To make the chimichurri sauce, take a large non-reactive bowl and pour ¾ cups of sherry vinegar into it.
Add ½ cup of olive oil to the vinegar and stir in ¾ cup of chopped fresh flat-leaf parsley, 3 tablespoons of freshly chopped oregano, 6 large cloves garlic (chopped), ½ teaspoon of red pepper flakes and ¾ teaspoon of salt.
Mix all the ingredients thoroughly, taste and adjust the amount of seasonings accordingly. Your chimichurri sauce is now ready.
Thoroughly wash the steaks and pat dry them with a paper towel.
Pour 1/3 cup of chimichurri sauce in a small bowl and pour the remaining sauce into a large baking dish.
Place the steaks in the dish containing chimichurri and coat both their sides with the sauce.
Cover the dish with a plastic sheet and place it in the refrigerator for two to six hours (the more the better).
Heat your charcoal or gas grill over high heat.
Grill the steaks directly over the grill, turning and brushing them with the marinade from the dish. Grill each side for 3 to 5 minutes for medium-rare.
Allow the steak to rest for a few minutes before slicing it for serving.
Serve with the reserved chimichurri sauce.