Preheat the oven to 375°F.
Pat the chicken breasts dry. Cut a pocket into the side of each chicken breast without slicing all the way through.
Season the chicken with salt, black pepper, garlic powder, Italian seasoning, and paprika.
In a bowl, mix cream cheese, spinach, mozzarella, parmesan, minced garlic, and red pepper flakes if using.
Spoon the spinach filling into each chicken pocket. Secure with toothpicks if needed.
Heat olive oil in an oven-safe skillet over medium-high heat.
Sear the stuffed chicken for 2–3 minutes per side until lightly golden.
Transfer the skillet to the oven and bake for 18–22 minutes, or until the chicken reaches 165°F internally.
Rest for 5 minutes, remove toothpicks, and serve warm.