Prepare the spinach according to the package directions and squeeze out as much moisture as you can from it.
Add it to a mixing bowl, along with the feta, cream cheese, garlic, onion powder, parsley, oregano, basil, salt and pepper, and mix well to combine. Set it aside.
Carefully cut a pocket into each chicken breast. To do so, place each breast flat side down on a cutting board, and press it down with a large spoon or spatula while you cut a slit lengthwise into the side, making sure not to cut all the way through to the other side. (Think pita pocket.)
Stuff each breast with as much of the filling as you can (if you have some left over, add it to a small, oven-safe bowl to make some fabulous cheese dip). Season each breast with salt and pepper.
In a large, oven-safe skillet, heat the olive oil over high heat. Once it’s hot, add the stuffed chicken breasts and cook for five minutes on one side. Flip them over, and transfer the skillet to the oven. Bake for 10 to 20 minutes, checking the internal temperature of the breasts after the first 10 minutes. Once the chicken reaches 165 degrees, remove it from the oven and serve. (If you made a side of the extra filling, just bake it until it’s hot all the way through and slightly browned on top.)