Preheat oven to 425°F and line a baking sheet with parchment paper.
Heat a sauté pan over medium heat. Spray with avocado oil spray and add spinach. Wilt for 30 seconds to 1 minute, stirring constantly, then remove from heat.
In a mixing bowl, combine Neufchatel cheese, plain Greek yogurt, shredded mozzarella cheese, sautéed spinach, sun-dried tomatoes, minced garlic, fresh basil and seasonings. Mix with rubber spatula until well combined.
Lay chicken breast flat on the cutting board and hold your knife parallel to the cutting board. Cut a slit about 3/4 of the way through the chicken breast. Lay chicken breasts flat on the cutting board cut side up. Cover chicken breasts with plastic wrap and pound out with a meat mallet until chicken is an even thickness.
Season the inside of the chicken breasts with onion powder, Italian seasoning, salt and black pepper. Scoop 1/4 of the filling onto the bottom portion of the chicken breast. Spread out into an even layer, then fold the top of the chicken breast over the filling.
Spray or brush the outside of the chicken with olive oil. Season with onion powder, Italian seasoning, salt and black pepper.
Bake stuffed chicken breasts at 425°F for 25-30 minutes, or until the internal temperature of the chicken reaches 165°F.