Here’s the plan: Take all of your leftover Christmas meats and make a batch of my guasacaca and tell everyone to drizzle it over everything — meat, chicken, fish — even vegetables and potatoes! Bask in their immediate adulation as you become a neighborhood hero. (Then tell everyone to buy my book.)
Put everything except the olive oil into a food processor and process until mostly smooth. Add the olive oil in a stream with the processor running and process until smooth. Let stand at room temperature for at least 1 hour for the flavors to blend. Taste and adjust seasoning.
Meanwhile, heat grill to high. Generously sprinkle flank steak with salt and ground black pepper. Place steak on grill and cook for 5 minutes on each side for medium-rare doneness.
Serve sauce at room temperature with skirt steak. Cover leftover sauce and store in the refrigerator (bring to room temperature before serving).