I'm not usually a fan of steak sauces. Whether they be bottled condiments, rich emulsifications, or simple pan reductions, I find that most things slopped onto a nicely charred slab of meat do more to mask, rather than enhance, the great beefy flavor of the steak. Salt, pepper, and beef is a good enough flavor combination for me on most days. But, I do make an exception for chimichurri—the Argentinian sauce of chopped parsley, garlic, olive oil and vinegar that brightens the flavor of grilled and charred meats without overpowering them.
In its most basic form, the bright green flavors of chimichurri usually stem from fresh parsley and fresh oregano, but I like to add some cilantro and green onion for some extra depth and bite, as well as some lemon zest to heighten the zip of the red wine vinegar in the sauce.
And although chimichurri is a welcome addition to most, if not all, cuts of beef, I like to marinate skirt steak in the green sauce, then quickly char the steak under a broiler (or slap it on the grill if the weather is right). After a quick rest, the steak is sliced and shingled into a soft sandwich roll, slathered with more chimichurri, and then served with even more of the sauce on the side for an Argentinian dip sandwich of sorts. Beef, bread, and chimichurri is really all you need for this simple sandwich—you'll find yourself plunging it into the sauce before every bite.