In a shallow bowl, combine the sherry wine, sherry vinegar, red onions, garlic and olive oil. Stir. Lay the steak over the marinade and turn it over. Wrap loosely and refrigerate overnight. Remove the steak from the marinade and season with the salt and pepper. Grill the steak over high heat for 4 to 5 minutes per side. Let rest for 5 minutes before thinly slicing.
In the bowl of a food processor, combine the olive oil, sherry vinegar, lemon juice, parsley, basil, oregano, garlic and shallots. Pulse until well blended but do not puree. Add the black pepper, the salt and crushed red pepper flakes. Transfer the sauce to a bowl and cover with plastic wrap for at least 2 hours, and up to 6 hours at room temperature. Chimichurri sauce will keep refrigerated in a well sealed container for up to 3 days.
For the yucca fries:
Place the vegetable oil in a 6-quart Dutch oven and heat to 325 degrees F using a candy thermometer to register the temperature.
While the oil is heating, peel the yucca and cut into wedges about 5 inches long and 1-inch wide. Place the yucca wedges in ice water to soak. When the oil reaches the desired temperature, remove the yucca from the ice water and pat dry with kitchen towels. Fry in batches in the hot oil, being sure to raise the heat as you add the yucca. Fry the yucca until they are soft, about 3 to 4 minutes. Remove from the oil using a slotted spoon and set aside on a paper-lined platter to drain. Repeat with the remaining yucca.
Once all the yucca are fried, raise the temperature of the oil to 350 degrees F.
Return the fried yucca to the oil and fry until crisp, golden brown, and puffed, 1 to 1 1/2 minutes. Remove the yucca fries from the oil using a slotted spoon and season with the salt and pepper. Repeat the process with the remaining yucca.