Place the flank steak on a large cutting board between two sheets of plastic wrap and pound thin (about ¼-inch thick) with a meat mallet, then transfer to a gallon-sized resealable bag.
In a small mixing bowl, whisk together the marinade ingredients and pour into the bag with the flank steak. Seal and shake around to coat. Refrigerate for at least 1 hour.
When ready to cook, heat an outdoor grill or indoor grill pan to medium-high heat.
Lay the marinated flank steak flat on a large cutting board and season both sides with salt, pepper, and garlic powder.
Spread the cheese wedges all over the top surface of the meat, then sprinkle the Italian seasoning over the cheese. Spread the spinach and sundried tomatoes over the Italian seasoning.
Starting with the longer edge, tightly roll the flank steak up and tie it with seven pieces of butcher’s twine, making eight 1-inch pinwheels. Insert a soaked skewer into each 1-inch pinwheel, to keep them intact after being sliced.
Slice between the twine, making six equal-sized pinwheels and two uneven end pieces.
Grease the grill, and cook each pinwheel for 2-4 minutes on each side, or until the desired doneness.
Let rest for 5 minutes, then remove the skewer and twine before serving.