Preheat your oven to 400°F.
Sauté the fresh spinach in a dry pan over medium heat until completely wilted, then squeeze out all excess moisture using a paper towel.
In a small mixing bowl, combine the wilted spinach, feta cheese, cream cheese, and dried oregano until well incorporated.
Slice a deep pocket into the side of the chicken breast, being careful not to cut all the way through.
Stuff the spinach and cheese mixture into the chicken pocket and secure with toothpicks if necessary.
Season the outside of the chicken breast evenly with sea salt, black pepper, and garlic powder.
Heat olive oil in an oven-safe skillet over medium-high heat and sear the chicken for 3 minutes per side until golden brown.
Place the skillet in the oven and bake for 12 to 15 minutes or until the internal temperature reaches 165°F.