Preheat the oven to 375°F (190°C).

Lay the flank steak flat and slice it horizontally to create a pocket for stuffing.

In a mixing bowl, combine chopped spinach, ricotta cheese, shredded mozzarella, sun-dried tomatoes, minced garlic, dried oregano, salt, and pepper. Stir until well blended.

Fill the pocket of the flank steak with the filling and roll it tightly; secure it with kitchen twine.

Heat olive oil in a skillet over medium-high heat and sear the stuffed flank steak for 4-5 minutes on each side.

Transfer the skillet to the preheated oven and bake for 20-25 minutes until the internal temperature reaches 130°F for medium-rare.

Let the steak rest for 10 minutes before slicing into 1-inch thick rounds.