Place the thawed spinach into the center of a tea towel and use it to wring out all of the moisture you can get. You don't want to water down the filling with the trapped moisture in the spinach. Place the dried spinach in the bottom of a medium bowl.
Add the cream cheese, parmesan, mozzarella, green onion, garlic, red pepper flakes, ½ teaspoon of salt, and a ½ teaspoon of pepper to the spinach and mix until evenly and well combined; set aside.
Use a sharp knife to cut a pocket into each chicken breast horizontally, making sure to be careful and not cut all the way through.
Stuff ¼ of the filling into the pocket of each chicken breast, making sure you can fully close the chicken around the filling.
Season the outside chicken breasts with the remaining 1 teaspoon of salt and ½ teaspoon of pepper.
Heat a large ovenproof skillet over medium high heat. Once the pan is hot add the oil. When the oil shimmers and is hot, saute the chicken breast in batches (so you don’t crowd the pan) for 4-5 minutes on each side or until golden brown.
Place all chicken in the skillet and place the skillet in the preheated oven and cook for another 10-12 minutes, or until a thermometer stuck into the thickest part of the meat registers 165℉.
Remove chicken to a serving platter or cutting board and let rest for 5 minutes before serving.
Notes
Leftovers can be stored in an airtight container in the refrigerator for up to 5 days.Reheat in a 350℉ until the chicken has reached 165℉.