Preheat oven to 425˚F with a rack in the center.
In a mixing bowl, mash together cream cheese, mayo and garlic with a fork then stir in mozzarella, parmesan and chopped spinach leaves until combined.
Lay chicken on a flat surface and cut slit ¾ of the way through without cutting all the way through.
Stuff each chicken breast with ⅓ of the mixture and seal horizontally with a toothpick (make sure the chicken breast can lay flat on the sides for searing).
Season the outside of the chicken breast all over with ¾ tsp salt, ½ tsp pepper, ½ tsp garlic powder and ½ tsp paprika, or season to taste.
Heat 1 ½ Tbsp olive oil in an oven-proof pan such as cast iron, or large heavy-bottomed skillet over medium heat.
Add chicken and sauté for 4 minutes per side until golden brown on each side.
Flip a second time and immediately transfer skillet to preheated oven.
Bake at 425˚F for 10-15 minutes until thermometer inserted into the thickest part of the chicken registers at 165˚F.
Rest chicken 5 minutes before slicing.
Spoon pan juices over chicken to serve.
