If using fresh spinach, in a large bowl, microwave spinach for 2-3 minutes or until wilted. Set aside to cool. If using frozen spinach, defrost completely. Squeeze excess moisture from spinach.
In a small bowl, combine mozzarella, cream cheese, parmesan, and garlic. Add drained spinach and mix to combine. Set aside.
On a cutting board, cut a horizontal slit in each chicken breast, creating a pocket. Place ¼ of the spinach mixture inside of each chicken breast pocket.
Use 2-4 toothpicks, positioned horizontally, to close each chicken breast pocket. Season chicken breasts on both sides with salt and pepper.
Heat a 12 inch cast iron skillet, or other skillet, on medium high heat, and add butter. Once butter is melted and skillet is warm, sear both sides of chicken breasts until browned, about 4-5 minutes each side.
Place skillet in oven and bake for 15-20 minutes, or until chicken reaches 165°F. Remove toothpicks before serving.
Notes
Count how many toothpicks you add and make sure to remove them all.
Use the thermometer to check the temperature of the thickest part of the chicken for doneness, not the filling. It should read 165°F.
Filling may seep out as the chicken bakes and is fine to serve along with the chicken.
Leftovers may be stored up to 3 days in the refrigerator, and up to 4 weeks in the freezer.