Pat the chicken breasts dry. Place each breast on a cutting board and carefully slice a horizontal pocket through the thickest side, stopping before you cut all the way through. Season both sides and the inside with salt and pepper.
Melt the butter in a large skillet over medium heat. Add the minced garlic and cook 30–45 seconds until fragrant. Add the spinach and stir until fully wilted, about 2–3 minutes. Transfer to a bowl and let cool 2 minutes. Stir in the cream cheese, Parmesan, red pepper flakes (if using), and Italian seasoning until smooth.
Divide the filling evenly among the four pockets. Press the edges gently closed. If needed, secure with toothpicks.
Heat the olive oil in the same skillet over medium-high heat. Place the stuffed chicken in the pan and sear 3–4 minutes per side until golden. Do not move the chicken too early so a good crust forms.
Preheat the oven to 375°F (190°C). Transfer the skillet (or move the chicken to a baking dish) and bake 15–20 minutes, or until the internal temperature reaches 165°F (74°C) in the thickest part of the meat. Rest 5 minutes before slicing.
Remove toothpicks if used. Slice crosswise to show the green filling and serve warm.