Preheat an oven broiler on high heat.

Open the sandwich rolls and spread each with about 1 teaspoon of softened butter. Sprinkle each buttered sandwich roll with about 1/2 teaspoon garlic powder.

Broil the sandwich rolls about 5 minutes, until the edges are slightly charred. Set aside.

Heat a large, heavy skillet over high heat, and add the vegetable oil. When oil just begins to smoke, drop in the flat iron steaks and quickly sear until steaks are browned, about 3 minutes. Flip the steaks and sear the other side for another 3 minutes; remove steaks immediately, or lower the burner temperature and cook to the desired degree of doneness. Set steaks aside and keep warm.

Assemble each sandwich roll with spinach leaves, tomato and avocado slices on one side, and 2 slices of mozzarella cheese on the other side. Sprinkle the tomato and avocado side with about 1/2 teaspoon of balsamic vinegar, and place a hot browned steak on the mozzarella cheese. Fold each sandwich closed to serve.