Preheat oven to 425°F and line a baking sheet with parchment paper.
Sauté spinach over medium heat until wilted, about 1 minute, then remove from heat.
In a bowl, mix neufchatel cheese, Greek yogurt, mozzarella, spinach, sun-dried tomatoes, garlic, basil, onion powder, Italian seasoning, salt, and pepper.
Cut a pocket into each chicken breast, season inside with salt and pepper, and fill with the cheese mixture.
Fold chicken closed, brush with olive oil, and season outside.
Bake for 25–30 minutes or until internal temperature reaches 165°F.