**Preheat the oven** to 375°F (190°C).
**Prepare the filling:** In a mixing bowl, combine cream cheese, mozzarella, Parmesan, spinach, garlic, and red pepper flakes. Mix until smooth.
**Butterfly the chicken:** Carefully cut a pocket into the side of each chicken breast without cutting all the way through.
**Stuff the chicken:** Fill each breast with the spinach-cheese mixture. Secure the opening with toothpicks or kitchen twine.
**Season and sear:** Season the outside of the chicken with salt, pepper, paprika, and garlic powder. Heat olive oil in an oven-safe skillet over medium heat. Sear the chicken on both sides until golden, about 3–4 minutes per side.
**Bake:** Transfer the skillet to the preheated oven and bake for 15–18 minutes, or until the internal temperature reaches 165°F (74°C).
**Serve:** Let rest for 5 minutes, then slice and serve with your favorite side.