In a medium bowl, mix the chopped spinach, ricotta cheese, grated Parmesan, mozzarella, minced garlic, oregano, salt, and pepper until well combined.
Take each chicken breast and carefully make a pocket by slicing horizontally with a sharp knife, ensuring not to cut all the way through.
Stuff each chicken breast with the spinach and ricotta mixture, using toothpicks to secure the opening, if necessary.
In a large oven-safe skillet, heat olive oil over medium heat. Once hot, add the stuffed chicken breasts and sear for about 3-4 minutes on each side until golden brown.
Transfer the skillet to the preheated oven and bake for about 20-25 minutes, or until the chicken reaches an internal temperature of 165°F (75°C).
Remove the skillet from the oven and let the chicken rest for 5 minutes.
Serve the stuffed chicken breasts drizzled with marinara sauce and garnish with fresh basil leaves.