Preheat the oven to 375°F.
Lay the flank steak flat on a cutting board and pound it to an even thickness using a meat mallet.
In a mixing bowl, combine the spinach, ricotta cheese, Parmesan cheese, sun-dried tomatoes, basil, garlic, salt, pepper, and Italian seasoning. Mix until well combined.
Spread the filling evenly over the flank steak, leaving a small border around the edges.
Starting from one end, tightly roll the steak up around the filling.
Secure the roll with kitchen twine or toothpicks at intervals.
Heat the olive oil in a large oven-safe skillet over medium-high heat.
Sear the stuffed flank steak on all sides until browned, about 3-4 minutes per side.
Transfer the skillet to the preheated oven and bake for 25-30 minutes, or until the internal temperature reaches 145°F for medium-rare.
Remove from the oven and let rest for 10 minutes before slicing. Serve warm.