Preheat the oven to 450 degrees F.
In a large sauté pan over medium heat, add 1 tablespoon of olive oil. Add the contents of the 8 oz. bag of fresh spinach and use a wooden spoon to gently stir for 2-3 minutes until all spinach is wilted. If the pan is too small to hold all of the spinach, start with half a bag, allow it to wilt, and add the remaining spinach.
Once all spinach has wilted, remove from heat and spoon into a colander. Press out as much of the excess liquid as possible.
In a medium-sized bowl, add the cooked spinach, feta, cream cheese, garlic and salt.
Place the chicken breasts on a cutting board. Use a large, flat spatula to press down on the top of the chicken breast and use your other hand to insert a knife into the thickest side of the chicken. Cut a pocket, being careful not to cut all the way through the other side and stopping before you get to the end of the chicken (creating a pocket, not a flap).
Season the chicken breasts with salt and pepper.
Place about a ½ cup of the spinach cheese mixture into each chicken pocket.
Heat 1 tablespoon of olive oil in a pan (preferably oven-safe) on medium-high heat. After the pan is heated through, add the chicken breasts – top side down. Allow to cook for 5 minutes without touching.
If you used an oven-safe pan, simply place in the oven for 10-15 minutes until chicken has cooked through. If not using an oven-safe pan, simply transfer the chicken breasts to an oven-safe dish and bake. If your chicken breasts are quite large – they will need the full 15 minutes.
10. Once finished baking, allow the chicken to rest for 5 minutes and serve.