Heat broiler with rack set 4" from heat. Line a rimmed baking sheet with aluminum foil; set aside. Place spinach in a clean kitchen towel, and squeeze out as much liquid as possible.
In a medium bowl, combine spinach,parsley, cheese, capers, vinegar, garlic, and red-pepper flakes. Season with salt and pepper; toss to combine. Cut and open steak by laying meatl on a cutting board, with the grain of the steak running horizontally. Using a long sharp knife, cut the meat almost in half horizontally, opening it llike a book but stopping before going all the way through.
Cut several 12" pices of kitchen string twine; space evenly underneath steak. Spread spinach mixture down center of meat. Roll steak over mixture, pressing firmly to compact the filling as much as possible, tie twine to secure roll. Cut roll in half crosswise. Place halves, seam side down, on prepared baking sheet, rub with oil, and season with salt and pepper.
Broil until browned, 8-10 minutes for medium-rare; let rest, loosely covered with foil, 10 minutes. Remove string, and slice steak into 1/2"rounds.