Preheat oven to 350°F.
Place lengths of kitchen twine about 1 inch apart under the butterflied flank steak, allowing ends to extend about 2 inches on either side. Generously season the steak with salt and pepper.
Mix olive oil, balsamic vinegar and garlic together in a small container and brush it over the top of the flank steak.
Layer the shredded mozzarella cheese, spinach leaves, and diced tomatoes over the open flank steak, leaving about ½ to ¾ of an inch uncovered around the edges. The stuffing will spread as you roll.
Roll the flank steak, tucking in the ingredients and rolling as you go. Roll with the grain.
When you're finished rolling, tie the twine at one-inch intervals.
Use a sharp knife to slice the roll between each piece of twine so that each pinwheel is about 1 inch in height. Cut across the grain of the flank steak. If necessary, trim the ends after you roll.
Place the steak pinwheels in a baking dish with the twine in place. Brush the remaining marinade on the outsides of the pinwheels.
Bake, uncovered, for 25-30 minutes or until the internal temperature reaches 145°F, for medium-rare steak.Note: The temperature will rise about 5 degrees once they are removed from the oven.
Allow the meat to rest for about 5 minutes so the juices redistribute. Once they are plated, snip off the string and enjoy.
