Prep the oven and pan. Preheat your oven to 400°F (200°C). Set a large oven-safe skillet on the stove. If you don’t have one, use any skillet for searing and transfer to a baking dish later.

Season the chicken. Pat the chicken dry. Sprinkle both sides with 1 teaspoon salt, 1/2 teaspoon black pepper, paprika, and onion powder. This builds flavor from the start.

Make the filling. In a bowl, mix softened cream cheese, Parmesan, mozzarella, garlic, lemon zest, red pepper flakes, and a pinch of salt and pepper until smooth.

Cook the spinach. Heat 1 tablespoon olive oil over medium heat. Add chopped spinach with a pinch of salt. Cook 1–2 minutes until just wilted. Let cool slightly, then stir into the cheese mixture. If using frozen spinach, squeeze it very dry and skip sautéing.

Cut the pockets. Place each chicken breast flat on a cutting board. Holding your knife parallel to the board, slice a pocket into the thickest side, stopping about 1/2 inch from the edges. Don’t cut through.

Fill and secure. Spoon the spinach mixture into each pocket, dividing evenly. Don’t overstuff. Secure the opening with toothpicks or tie with short lengths of kitchen twine so the filling stays put.

Sear for color. Heat remaining 1 tablespoon olive oil and the butter in the skillet over medium-high. When hot and shimmering, add chicken. Sear 2–3 minutes per side until golden. Work in batches if needed to avoid overcrowding.

Bake to finish. Transfer the skillet to the oven (or move chicken to a baking dish). Bake 12–16 minutes, depending on thickness, until the internal temperature reaches 165°F (74°C) in the thickest part.

Rest and serve. Let the chicken rest 5 minutes. Remove toothpicks before slicing. Spoon any pan juices over the top and serve.