Preheat your oven to 375°F (190°C).
Place the flank steak between two pieces of plastic wrap and pound it to an even thickness of about ½ inch using a meat mallet or rolling pin.
In a mixing bowl, combine the chopped spinach, ricotta cheese, mozzarella cheese, minced garlic, salt, pepper, and Italian seasoning. Mix well until all ingredients are evenly combined.
Spread the cheese and spinach mixture evenly over the pounded flank steak, leaving a small border around the edges.
Starting from one end, carefully roll the flank steak tightly into a log shape. Secure the ends with toothpicks or tie with kitchen twine.
Heat olive oil in a large oven-safe skillet over medium-high heat. Once hot, sear the rolled flank steak for about 3-4 minutes on each side until browned.
Drizzle balsamic vinegar over the top of the steak, then transfer the skillet to the preheated oven. Bake for 20-25 minutes, or until the internal temperature reaches 145°F (medium-rare).
Remove from the oven and let the pinwheels rest for 5-10 minutes before slicing into 1-inch thick pieces. Serve warm.