Preheat your oven to 450 degrees F (232 degrees C).

Pat the chicken breasts dry with paper towels. Season both sides with salt and pepper. Use a sharp knife to carefully cut a horizontal slit in each breast, creating a deep pocket without cutting all the way through. Set aside.

Place the spinach in a large microwave-safe bowl. Microwave for 2 to 3 minutes until wilted, or saute on the stove over medium heat for 3 to 5 minutes. Set aside to cool until comfortable to handle.

In a separate large bowl, mash together the softened cream cheese and minced garlic until smooth. Stir in 1/2 cup of the shredded mozzarella.

Once the spinach has cooled, squeeze it firmly over the sink to remove as much water as possible. Stir the squeezed spinach into the cream cheese mixture.

Spoon the spinach mixture evenly into each chicken breast pocket. Use toothpicks positioned horizontally to keep the filling secure during cooking.

Heat olive oil in a large cast iron skillet (at least 12 inches) over medium-high heat until shimmering. Add the stuffed chicken and sear for 4 to 5 minutes per side until deeply golden brown on both sides.

Working quickly, top each chicken breast with 2 tomato slices and 2 tablespoons of the remaining shredded mozzarella.

Transfer the skillet to the preheated oven. Bake for about 15 minutes, until the internal temperature reaches 165 degrees F when measured in the thickest part of the chicken (not the filling). If you want the cheese more golden, broil for 1 to 3 minutes at the end.

Remove from the oven and let the chicken rest for 3 to 5 minutes. Remove toothpicks before serving.