Combine the marinade ingredients in a small bowl. Butterfly the flank steak by slicing through it horizontally, leaving the meat attached at one end. Open it out, cut side up. Season with salt and pepper.
Stuffing:Lay the slices of prosciutto over the cut side of the steak. Lay the leaves of spinach over the slices. In a small bowl, combine the basil, parsley, bread crumbs, garlic, cheese and olive oil. Spread over the spinach leaves and roll the steak up tightly like a jelly roll. Tie the roll in several places and brush the outside all over with the marinade.
To Cook on a grill:On a charcoal grill, sear the matambre on all sides directly over the coals for about 5 to 6 minutes. Move the meat to an area with no coals and then cook with the cover on for another 10 to 15 minutes, or until the meat is rare to medium rare, 120 to 130 deg F. Remove the meat from the grill and cover loosely with foil, and let rest for 10 to 20 minutes. The internal temperature should be 130 to 135 deg.
To Roast in oven:Preheat the oven to 350 deg. Heat 1 Tablespoon of oil in a skillet over medium high heat. Sear the beef roll on all sides, 6 to 8 minutes total. Transfer the meat to a roasting pan, and roast for 20 to 30 minutes, and check the internal temperature. When the meat reaches 120 to 130 deg F, remove from oven, tent with foil and let the matambre rest for 10 to 20 minutes.
Sauce:In a food processor, process the almonds or pine nuts to a thick paste. Add the remaining ingredients and process until smooth. Serve alongside the meat.