In a medium bowl, add cream cheese, Parmesan cheese, mayo, spinach, garlic, 1/2 tsp salt and red pepper flakes. Stir with a fork until well combined. Refrigerate.
In a small bowl, combine smoked paprika, garlic powder, 1/4 tsp salt and pepper. Stir with a fork and set aside.
With a sharp filet knife, slice each chicken breast in half but not through, creating a pocket. If you unfold it, it looks like a heart. Sprinkle with seasonings from previous step on both sides.
Stuff each breast with spinach mixture. You can secure it with a toothpick.
Preheat cast iron skillet on medium-high heat and swirl oil to coat. Add stuffed chicken breasts and cook for 5 minutes. Carefully flip, reduce heat to low and cook for another 5 minutes. Then cover with a lid and cook until internal temperature reaches 165 degrees F.
Serve warm or cold with juices from the bottom of the pan.
Notes
Store and reheat: Refrigerate leftovers in an airtight container for up to 5 days. Simmer with a splash of broth, in a skillet, on low while covered for 5-10 minutes.
Freeze: Freeze in an airtight container for up to 3 months. Thaw in the fridge overnight and reheat as per instructions above.
Frozen spinach: Replace fresh spinach with 1/2 cup thawed and well squeezed previously frozen spinach.