2 packages (10 ounces each) frozen leaf spinach, thawed
1/4 cup fresh parsley, chopped
1/4 cup grated Asiago cheese
2 tablespoons balsamic vinegar
1 garlic clove, minced
1/4 teaspoon red pepper flakes
Sea salt and fresh ground pepper to taste
1 flank steak (1- 1 1/2 pounds)
1 teaspoon olive oil
Heat broiler, with rack set 4 inches 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 mixing bowl, combine spinach, parsley, cheese, vinegar, garlic, and red-pepper flakes. Season with salt and pepper; toss to combine.
Butterfly the Steak:
Lay the meat 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 like a book but stopping before going all the way through.
Cut several 12-inch pieces of kitchen 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 to 10 minutes for medium-rare; let rest, loosely covered with foil, 10 minutes. Remove string, and slice steak into 1/2-inch rounds.