Place each chicken breast on a cutting board and carefully cut a pocket into the thick side using a sharp boning knife, making sure not to cut all the way through.
Pound the chicken breasts gently with a meat mallet to an even thickness of about 1.5 inches. This helps with uniform cooking and makes stuffing easier.
In a bowl, mix the chopped herbs, minced garlic, cheese, spinach (if using), toasted nuts, and a pinch of salt and pepper to create the flavorful filling.
Using a spoon or spatula, carefully stuff each chicken pocket with the herb and cheese mixture, being careful not to overfill. Use toothpicks or kitchen twine to seal the opening if needed.
Heat olive oil in an ovenproof skillet over medium-high heat until shimmering. Sear the stuffed chicken breasts for 2-3 minutes per side until golden and crispy.
Transfer the skillet to a preheated oven at 190°C (375°F) and roast for 20-25 minutes, or until the internal temperature reaches 74°C (165°F) and the filling is bubbling.
Remove the chicken from the oven, tent loosely with foil, and let rest for 5 minutes to allow the juices to settle.
Carefully remove toothpicks or twine, then slice the chicken to reveal the melted cheese and flavorful stuffing inside.
Serve hot, enjoying the juicy, cheesy filling with a crispy exterior and tender chicken.
