Preheat your oven to 400°F (200°C). Lightly grease a 9×13-inch baking dish with olive oil or cooking spray to prevent sticking.
2 tablespoons olive oil
Lay each chicken breast flat on a cutting board. Using a sharp knife, carefully cut a horizontal pocket into the thickest side of the breast, going about three-quarters of the way through without cutting all the way through to the other side.
4 large boneless, skinless chicken breasts
Pat the chicken dry with paper towels. In a small bowl, combine 1 teaspoon of the kosher salt, 1/2 teaspoon black pepper, garlic powder, onion powder, and smoked paprika. Rub this spice mixture all over the chicken breasts, including lightly inside the pockets. Set aside.
4 large boneless, skinless chicken breasts, 1 1/2 teaspoons kosher salt, 1/2 teaspoon black pepper, 1 teaspoon garlic powder, 1 teaspoon onion powder, 1 teaspoon smoked paprika
In a medium skillet, heat 1 tablespoon olive oil (or butter) over medium heat. Add the minced garlic and cook for about 30 seconds, just until fragrant. Add the spinach and cook, stirring, until wilted and bright green, 2–3 minutes. If using frozen spinach, warm it through with the garlic. Remove from heat and let cool slightly.
1 tablespoon olive oil, 3 cloves garlic, 4 cups fresh baby spinach
In a medium bowl, combine the cream cheese, mozzarella, 1/4 cup grated Parmesan, 1/2 teaspoon kosher salt, 1/4 teaspoon black pepper, crushed red pepper flakes (if using), and lemon zest (if using). Stir until mostly smooth, then fold in the slightly cooled spinach and garlic mixture until evenly combined. The filling should be thick and spreadable.
4 ounces cream cheese, 1/2 cup shredded mozzarella cheese, 1/4 cup grated Parmesan cheese, 1/2 teaspoon kosher salt, 1/4 teaspoon black pepper, 1/4 teaspoon crushed red pepper flakes, 1/2 lemon, 4 cups fresh baby spinach, 3 cloves garlic
Divide the spinach-cheese filling evenly among the chicken breasts, spooning it into the pockets and tucking it in. If desired, secure the openings with toothpicks to help keep the filling inside.
4 large boneless, skinless chicken breasts
Heat 1 tablespoon olive oil in a large oven-safe skillet over medium-high heat. When hot, add the stuffed chicken breasts and sear for 2–3 minutes per side until lightly golden. You are not cooking them through, just building flavor and color. If your skillet is not oven-safe, transfer the seared chicken to the prepared baking dish.
2 tablespoons olive oil, 4 large boneless, skinless chicken breasts
If not already in an oven-safe skillet, place the seared chicken in the prepared baking dish. Sprinkle the tops lightly with the additional 2 tablespoons grated Parmesan, if using. Bake at 400°F (200°C) for 20–25 minutes, or until the thickest part of the chicken reaches 165°F (74°C) on an instant-read thermometer.
2 tablespoons grated Parmesan cheese, 4 large boneless, skinless chicken breasts
Remove the chicken from the oven and let rest for 5–7 minutes to allow the juices to redistribute. Garnish with chopped fresh parsley and serve with lemon wedges, if desired.
fresh parsley, lemon wedges