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.
In medium-size bowl, combine chopped spinach, blue cheese, bell pepper, tomatoes, bread crumbs, egg yolk, 1/4 teaspoon each of the garlic salt and the pepper.
Season steak with an additional 1/4 teaspoon each of the garlic salt and pepper. 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 left to right. 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 1/4 teaspoon each garlic salt and pepper.
Roast at 425 degrees 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.