Preheat oven to 375°F (190°C). Pat chicken dry and slice a pocket in each breast.
Season pockets with salt, pepper, garlic powder, and Italian seasoning.
Stuff each pocket with spinach, a slice of mozzarella, and prosciutto; sprinkle Parmesan inside. Secure with toothpicks.
Heat olive oil and butter in oven-safe skillet over medium-high. Sear chicken 2–3 minutes per side until golden.
Transfer skillet to oven and bake 15–20 minutes until internal temp reaches 165°F (74°C).
Rest 5 minutes, remove toothpicks, slice, and serve.