1 1/4 pounds flank steak
2 bunches fresh spinach, steamed, drained and squeezed
1/4 cup packed fresh flat- leaf parsley leaves, coarsely chopped
1/2 cup mushrooms
2 tablespoons butter
1/2 cup grated Asiago cheese (4 ounces)
2 tablespoons capers, drained and rinsed
1 tablespoon Worcestershire sauce
1/4 teaspoon crushed red- pepper flakes
4 teaspoons olive oil
2 cloves garlic, minced
Coarse salt and freshly ground pepper
Heat butter and 2 tablespoons olive oil over medium heat. Add mushrooms and 1 clove minced garlic. Sauté until soft, about 5 minutes seasoning with salt and fresh ground pepper about halfway through.
Heat broiler, with rack set 4 inches from heat source. Line a rimmed baking sheet with aluminum foil. Squeeze spinach in a clean kitchen towel (or a double layer of paper towels) to remove as much liquid as possible. Combine spinach, mushrooms, parsley, cheese, capers, Worcestershire sauce, garlic, and red- pepper flakes in a bowl. Season with salt and pepper; toss to combine.
To butterfly steak, lay it on a cutting board with a short side facing you. Using a long knife, slice steak horizontally, leaving one side intact; open up like a book.
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 filling; tie twine to secure roll. Cut roll in half crosswise. Place halves, seam side down, on prepared baking sheet; rub with oil, dividing evenly, and season with salt and pepper.
Broil steak until browned on all sides, turning as needed with tongs, 8 to 10 minutes for medium- rare; let rest, covered loosely with aluminum foil, 10 minutes. Remove string;
slice steak 1/2 inch thick