Heat oven to 425 degrees F.
Lay steak on work surface. Holding sharp knife parallel to work surface and starting at a long side, slice flank steak in half to opposite long side, without cutting all the way through; open up the steak like a book. Flatten slightly to an even thickness.
Squeeze liquid from spinach; discard liquid. In medium-size bowl, combine spinach, cheese, peppers, and egg yolk.
Season open part of steak with 1 teaspoon salt. Press filling onto steak, leaving a 1-inch border on all sides. Roll up steak to enclose filling, beginning on a short side; the grain of the meat will be running from top to bottom. Tuck any loose filling back into ends.
Tie steak with cotton twine at 2-inch intervals to secure. Rub outside with oil, then sprinkle with remaining teaspoon salt.
Roast at 425 degrees F for 35 minutes, then increase heat to broil and broil for 10 minutes, turning once. Let meat rest 15 minutes. Remove twine, slice and serve.