Preheat your oven to 190°C (375°F).
Use a sharp boning knife to carefully slice a pocket into each chicken breast, making sure not to cut all the way through, leaving a flap of meat intact.
Season the outside and inside of each chicken pocket generously with salt, pepper, and your chosen herbs.
In a bowl, combine the shredded cheese, wilted spinach, sautéed garlic, lemon zest, and herbs, mixing until well combined.
Stuff each chicken pocket generously with the prepared cheese and spinach mixture, pressing lightly to secure the filling.
Heat olive oil in a skillet with an ovenproof handle over medium-high heat until shimmering and fragrant.
Place the stuffed chicken breasts in the skillet and sear for about 2-3 minutes per side, until golden brown and crispy.
Transfer the skillet to the preheated oven and bake for 20-25 minutes, or until a meat thermometer reads 75°C (165°F) when inserted into the thickest part.
Remove the skillet from the oven and let the chicken rest for 5 minutes, allowing juices to redistribute.
Slice the chicken breasts open to reveal the gooey, cheesy filling and serve hot, garnished with additional herbs if desired.