Make the creamy cheese filling: In a medium bowl, combine softened cream cheese, shredded mozzarella, grated parmesan, wilted chopped spinach, minced garlic, and red pepper flakes. Mix with a fork until smooth and thoroughly combined. Set aside.

Cut the chicken pockets: Place each chicken breast flat on a cutting board. Using a sharp chef or boning knife, carefully cut a deep horizontal pocket into the thick side of each breast, cutting about 3/4 of the way through without slicing all the way out. Season the inside of each pocket with a pinch of salt and pepper.

Stuff & secure: Spoon one-fourth of the creamy cheese mixture into each chicken pocket. Press the edges together to enclose the filling and secure with 2 to 3 wooden toothpicks inserted horizontally to keep the cheese locked inside.

Season the exterior: In a small ramekin, mix smoked paprika, Italian seasoning, garlic powder, remaining salt, and black pepper. Generously rub the spice mixture all over the outside of the chicken breasts.

Sear & cook to golden perfection: Heat olive oil and butter in a large oven-safe cast-iron skillet over medium-high heat. Add the stuffed chicken breasts and sear for 5 to 6 minutes without moving until a deep golden-brown crust forms. Carefully flip, spoon the garlic herb butter pan juices over the top, and cook on the second side for another 6 to 8 minutes (or transfer to a 375°F/190°C oven for 10 minutes) until the internal temperature reaches 165°F (74°C).

Rest & carve: Transfer chicken to a clean carving board and let rest for 5 minutes so the juices redistribute and the cheese firms slightly. Remove toothpicks, slice into thick medallions, and drizzle with warm pan drippings before serving!