Preheat your oven to 200°C (390°F). Place a rack inside a baking sheet or use an oven-safe skillet.
Using a sharp knife, slice into the thick side of each chicken breast to create a pocket, being careful not to cut all the way through.
Heat a tablespoon of olive oil in a skillet over medium heat. Add minced garlic and cook until fragrant, about 30 seconds, until it releases a pleasant aroma.
Add chopped spinach to the skillet, stirring until wilted and bright green, about 2 minutes. Remove from heat and let cool slightly.
Chop sun-dried tomatoes into small pieces and mix them into the spinach along with crumbled feta, lemon zest, and chopped herbs. Season with salt and pepper to taste. Stir everything together until well combined.
Using a small spoon or spatula, generously stuff each chicken pocket with the spinach mixture, pressing gently to pack the filling inside.
Secure each stuffed chicken breast with kitchen twine or toothpicks to keep the filling inside during cooking.
Heat the same skillet over medium-high heat and sear the stuffed chicken breasts, skin-side down if applicable, until golden brown and crispy, about 3-4 minutes. Flip and sear the other side for another 3 minutes.
Transfer the skillet to the preheated oven and bake for 15-20 minutes, or until the internal temperature reaches 75°C (165°F), and the chicken is cooked through.
Remove the skillet from the oven, cover loosely with foil, and let the chicken rest for 5 minutes to retain juices.
Carefully remove toothpicks or twine, then slice the chicken breasts to reveal the colorful, cheesy filling. Serve hot and enjoy the juicy, flavorful bites.