Preheat your oven to 375°F.

Lay the flank steak flat on a cutting board. Use a sharp knife to butterfly the steak by slicing it horizontally, but not all the way through, so it opens like a book.

In a mixing bowl, combine the chopped spinach, ricotta cheese, Parmesan cheese, sun-dried tomatoes, minced garlic, oregano, salt, and black pepper. Mix until well combined.

Spread the cheese and spinach mixture evenly over one side of the opened flank steak.

Carefully fold the other side of the steak over the filling. Use kitchen twine or toothpicks to secure the edges and prevent the filling from spilling out.

Heat olive oil in a large oven-safe skillet over medium-high heat. Sear the stuffed flank steak for about 3-4 minutes on each side until browned.

Drizzle the balsamic vinegar over the steak and transfer the skillet to the preheated oven. Bake for 20-25 minutes for medium-rare, or until the desired doneness is reached.

Remove the skillet from the oven and let the steak rest for 10 minutes before slicing.

Garnish with fresh basil leaves if desired, and serve.