Cut a 3-inch-wide pocket in the side of each chicken breast, being careful not to slice all of the way through. Pat the chicken dry with paper towels and season with salt and pepper on all sides.
In a large bowl, stir together the cream cheese, feta, spinach, garlic, basil, oregano, onion powder and ½ teaspoon kosher salt.
Stuff the cream cheese mixture into the pocket of each chicken breast.
Heat oil and butter in a large cast iron skillet over medium heat. Add the chicken breasts to the pan and cook just until golden brown on each side, about 3-4 minutes per side. Transfer the skillet to the oven and continue cooking for about 12-15 more minutes, or until the meat reaches an internal temperature of 165° F.
Remove chicken to a cutting board and let rest for about 5 minutes before serving.
Notes
Cooking for a smaller family? You can cook 2-4 chicken breasts, depending on how many mouths you need to feed. The filling should be the perfect amount for 4 chicken breasts. If you're preparing only 2 pieces of chicken, you can cut the filling ingredients in half since you won't need as much.
When cutting the pocket in the chicken, hold one hand on top of the chicken breast and slice horizontally into the thickest part of the middle of the breast with a very sharp knife, being careful not to cut all of the way through.
Squeeze the spinach very dry before adding it to the filling. I like to put it in a dish towel and wring it out over the sink. This will prevent the spinach from watering-down the creamy, cheesy filling.
If you don’t have a cast iron skillet, sear the chicken in a skillet first, and then transfer the meat to an oven-safe dish to finish baking it.
The total cooking time will vary depending on the size and thickness of your chicken. Use a meat thermometer to know exactly when the chicken is done. It should read 165° F on an instant-read thermometer.