Preheat the oven to 350°F.

In a bowl, combine the chopped spinach, ricotta cheese, Parmesan cheese, garlic powder, oregano, salt, and black pepper. Mix until well combined.

Lay the beef slices flat on a clean surface. Spoon a portion of the cheese and spinach mixture onto each slice, spreading it evenly but leaving a small border around the edges.

Starting from one end, roll each beef slice tightly around the filling. Secure the ends with toothpicks or tie with kitchen twine to keep the filling inside.

In a large oven-safe skillet, heat the olive oil over medium heat. Add the braciole and sear on all sides until browned, about 3-4 minutes per side.

Pour the marinara sauce over the seared braciole in the skillet. Cover the skillet with a lid or aluminum foil and transfer it to the preheated oven. Bake for 30-35 minutes, or until the beef is tender.

Remove from the oven and let rest for 5 minutes before removing toothpicks or twine. Slice and serve with additional marinara sauce if desired.