Preheat your oven to 190°C (375°F) and prepare a baking dish.
Use a sharp knife to slice each chicken breast horizontally, creating a pocket without cutting all the way through.
In a mixing bowl, combine softened cream cheese with chopped sun-dried tomatoes, sautéed spinach, crushed garlic, chopped herbs, and lemon zest if using. Mix until smooth and well incorporated.
Fill each chicken pocket generously with the cheese mixture, then secure with toothpicks to keep the stuffing inside during baking.
Sprinkle shredded cheese on top of the stuffed chicken breasts for extra gooeyness.
Place the stuffed chicken breasts in the oven-safe dish and bake for about 25-30 minutes, until the chicken is cooked through and golden on top. Internal temperature should reach 74°C (165°F).
Remove from the oven, let rest for a few minutes, then remove toothpicks and serve hot, enjoying the bubbly, cheesy interior with a crisp outer crust.