Preheat your oven to 375°F (190°C).
Slice each chicken breast horizontally to create a pocket.
Combine spinach, feta, cream cheese, Parmesan, garlic, oregano, salt, pepper, and red pepper flakes in a bowl.
Stuff the chicken breasts with the filling and secure if necessary.
Sear the stuffed chicken in olive oil for 3-4 minutes on each side until golden brown.
Transfer to the oven and bake for 25-30 minutes until the internal temperature reaches 165°F (75°C).
Let the chicken rest for 5 minutes before serving.