Preheat the oven to 375°F. Pat the chicken breasts dry and carefully cut a pocket into the thickest side of each breast, making sure not to cut all the way through.
Heat a small skillet over medium heat and sauté the onion and garlic for 1–2 minutes until fragrant. Add the spinach and cook until wilted. Let it cool slightly, then mix with cream cheese, mozzarella, Parmesan, and red pepper flakes until creamy.
Combine garlic powder, smoked paprika, Italian seasoning, salt, and black pepper. Rub the seasoning mixture evenly over the outside of each chicken breast.
Divide the spinach filling evenly between the chicken breasts, gently pressing it into the pockets
Heat olive oil in an oven-safe skillet over medium-high heat. Sear the stuffed chicken breasts for 3–4 minutes per side until lightly golden.
Transfer the skillet to the oven and bake for 18–22 minutes, or until the chicken reaches an internal temperature of 165°F and the cheese filling is melted and hot.
Remove the chicken from the oven and let it rest for 5 minutes before slicing. Carefully remove toothpicks, slice, and serve with your favorite sides.