In a bowl, mix together onion, spinach, sun dried tomatoes, mozzarella cheese, cream cheese, ricotta cheese, and 1/4 stick of butter.
Cut opening into side of chicken breast creating a pocket. Season to desire inside the pocket and outside the whole chicken breast.
Stuff chicken with mix.
Heat large cast-iron-skillet over medium heat and pour in sun dried tomato oil and butter. Place chicken in skillet and sear for 3 minutes on each side. Then place skillet into oven to bake for 30 minutes.