In a medium bowl, combine the chopped spinach, cream cheese, mozzarella, Parmesan, minced garlic, onion powder, salt, pepper, and Italian seasoning. Mix until well combined and set aside.
Place each chicken breast on a cutting board. Using a sharp knife, carefully slice each breast horizontally to create a pocket, being careful not to cut all the way through.
Generously fill each chicken breast pocket with the spinach mixture. Secure the openings with toothpicks if necessary, to prevent the stuffing from leaking out during cooking.
In a large oven-safe skillet, heat the olive oil over medium-high heat. Once hot, add the stuffed chicken breasts and sear for about 3-4 minutes on each side until golden brown.
Once seared, transfer the skillet to the preheated oven and bake for 25-30 minutes, or until the chicken is cooked through and reaches an internal temperature of 165°F (75°C).
Remove the skillet from the oven and let the chicken rest for 5 minutes before removing toothpicks. Garnish with fresh basil leaves if desired. Serve warm.