Flank steak, also known as London broil, is an inexpensive, tough cut of beef from the lower section of the cow. If it is cut very thin and tenderized, however, it can be the basis of delicious, elegant beef meals. Make a spinach and feta cheese stuffing and follow a few steps to transform a piece of flank steak into a tasty and visually appealing beef main dish that is appropriate for company.
Set a broiler rack 4 inches from the heat source. Preheat the broiler. Line a rimmed baking sheet with aluminum foil. Steam the spinach, and drain away all liquid. Gently squeeze the steamed spinach into a double layer of paper towels to remove any remaining liquid.
Put the spinach, chopped parsley, feta cheese, capers, vinegar, red pepper flakes and minced garlic into a bowl and mix well. Season with salt and pepper to taste. Stir briefly to combine. Set aside the spinach mixture.
Lay the meat on the cutting board with one long side facing you. Butterfly the meat by slicing it with a long knife; slice through the meat with the blade of the knife parallel to the cutting board. Cut through the meat to within 1/4 to 1/2 inch of the other long edge, leaving it uncut. Open up the meat like a book, with the uncut long side like the center book binding.
Cut two 12-inch pieces of kitchen twine. Slide them evenly under the butterflied steak. Spread the spinach mixture down the center of the meat to within 1 inch of each end; use all the filling mixture. Begin rolling the steak from one short end, pressing down gently to compact the filling. Roll up the steak completely, then tie the twine around the stuffed meat. Cut the steak roll in half between the two twine ties. Place each piece, seam side down, on a prepared baking sheet.
Brush both pieces of rolled steak with olive oil; sprinkle with additional salt and pepper. Broil the steak until it is evenly brown on all sides, turning the meat with tongs as needed during broiling. Broil eight to 10 minutes for medium-rare or 10 to 14 minutes for medium. Remove from the heat, cover the steak loosely with aluminum foil and allow it to sit for 10 minutes before serving. Cut both pieces of twine and discard them. Slice steak into 1/2-inch slices for serving.