In a shallow nonreactive bowl that is large enough to hold the steak, combine the sherry, sherry vinegar, red onion, garlic, and olive oil. Stir to blend well. Lay the steak over the marinade and turn so that it is coated on both sides. Wrap the bowl loosely in plastic wrap and refrigerate overnight, turning the steak occasionally.
Remove the steak from the marinade and season it with the salt and pepper. Discard the marinade.
Heat 1 tablespoon of the remaining olive oil in a large saute pan over medium-high heat. Place the steak in the pan and cook for 4 to 5 minutes per side over medium-high heat, for medium rare. Remove the steaks to a platter, tent with aluminum foil and let it rest for 5 minutes. Thinly slice the steak across the grain and serve it with the chimichurri sauce.
Combine the olive oil, vinegar, sherry, lemon juice, parsley, basil, garlic, red onion, cilantro and oregano in a food processor, and pulse until well blended. Cook's Note: Do not puree. Stir in the kosher salt, black pepper, and crushed red pepper.
Transfer the sauce to a nonreactive bowl, cover it with plastic wrap, and set it aside for at least 2 hours and up to 6 hours at room temperature. The sauce will keep in an airtight nonreactive container in the refrigerator for up to 3 days.