Make a flank steak stuffed with spinach by combining the spinach with parsley, cheese, capers, vinegar and spices and spreading the mixture over the steak. Cut the rolled steak in half, tie the halves with twine, and cut the halves into servings after cooking.
To make the filling for flank steaks, thaw 20 ounces of frozen leaf spinach, place it in a kitchen towel, and squeeze to remove the liquid. Combine the spinach in a bowl with 1/4 cup of fresh chopped parsley, 1/4 cup of grated Asiago cheese, 2 tablespoons of rinsed and drained capers and 2 tablespoons of balsamic vinegar. Add 1 clove of minced garlic, 1/4 teaspoon of red pepper flakes and salt and ground pepper to taste, and toss the mixture to combine.
Heat the broiler with the rack set 4 inches from the heat, and line a baking sheet with aluminum foil. Cut open a 1-pound flank steak, and spread the spinach mixture down the middle. Roll the steak, and tie several pieces of twine around it to secure the filling and keep the steak rolled. Cut the rolled steak in half, and place the halves on the baking sheet with the seams down. Rub the steak rolls with 1 tablespoon of olive oil, and season with salt and pepper to taste. Broil the steak rolls for eight to 10 minutes for medium rare, cover them with aluminum foil, and let them rest for 10 minutes. Remove the strings from the steak rolls, and slice them into 1/2-inch rounds before serving.