Preheat your oven to 450°F (232°C).
Pat the chicken breasts dry and season both sides with sea salt and black pepper. Using a sharp knife, cut a horizontal slit in each chicken breast to create a pocket. Set aside.
In a large bowl, steam the chopped spinach in the microwave for 2-3 minutes until wilted, or sauté it on the stove for 3-5 minutes. Once wilted, set the spinach aside to cool.
While the spinach cools, mash together the softened cream cheese and minced garlic in a separate large bowl. Stir in 1/2 cup of shredded mozzarella cheese.
When the spinach is cool enough to handle, squeeze out as much water as possible and mix it into the cream cheese mixture.
Stuff each chicken breast with the spinach and cheese mixture. Secure the filling inside with toothpicks positioned horizontally to close the pocket.
Heat the olive oil in a large cast iron skillet over medium-high heat. Sear the stuffed chicken breasts for 4-5 minutes per side, until golden brown.
Quickly top each chicken breast with 2 tomato slices and 2 tablespoons of shredded mozzarella cheese.
Transfer the skillet to the preheated oven and bake for 15 minutes, or until the internal temperature of the chicken reaches 165°F (74°C). Make sure to check the temperature in the chicken and not in the filling.
For a darker, more golden cheese topping, place the skillet under the broiler for 1-3 minutes (optional). Serve hot and enjoy!