Heat oven to 425 degrees. Lay steak on work surface. Holding sharp knife parallel to work surface and starting at a long side, slice flank steak in half without cutting all the way through; effectively butterflying the steak. Flatten slightly to an even thickness. In a skillet, cook the garlic and peppers in 1 tablespoon of olive oil until soft. Add the spinach and cook until wilted. Remove from the heat. In medium-size bowl, combine spinach, cheese, peppers, garlic, bread crumbs, egg yolk, 1/4 teaspoon of the garlic salt, and half the salt and pepper. Season steak with an additional 1/4 teaspoon of the garlic salt. Press filling onto steak, leaving a 1-inch border on all sides. Roll up steak to enclose filling, beginning on a short side. Tie steak with kitchen twine to secure. Rub outside with oil, then sprinkle with remaining salt and pepper. Roast for 35 minutes, then broil and broil for 5 minutes to brown. Let meat rest 15 minutes. Remove twine and slice.