Preheat and prep: Heat the oven to 375°F (190°C). Line a baking sheet with parchment or lightly grease a baking dish.
Make the filling: In a bowl, stir together the cream cheese, mozzarella, Parmesan, garlic, spinach, chives, lemon zest, lemon juice, and red pepper flakes.
Mix until smooth and evenly combined. Taste and adjust salt if needed.
Season the chicken: Pat the chicken breasts dry. In a small dish, combine salt, pepper, garlic powder, onion powder, and smoked paprika.
Sprinkle evenly over both sides of the chicken.
Cut a pocket: Lay each chicken breast flat. Using a sharp knife, carefully cut a deep pocket into the thickest side, keeping the knife parallel to the cutting board. Don’t slice all the way through.
Stuff the chicken: Spoon the cream cheese mixture into each pocket, dividing evenly.
Don’t overfill to the point of bursting. Secure the opening with toothpicks to help keep the filling inside.
Sear for color: Heat olive oil in a large oven-safe skillet over medium-high heat. Sear the chicken 2–3 minutes per side until golden.
If you don’t have an oven-safe skillet, transfer to the prepared baking dish after searing.
Bake to finish: Move the skillet to the oven and bake 15–20 minutes, or until the thickest part of the chicken reaches 165°F (74°C). The time will vary based on thickness.
Rest and serve: Let the chicken rest 5 minutes. Remove toothpicks, slice, and spoon any pan juices over the top.
Serve with a simple salad, sautéed green beans, or roasted potatoes.