Preheat oven to 375°F and spray a 9x13 baking dish with cooking spray.
Trim fatty pieces from chicken breasts and carefully cut a pocket through the center of each breast.
Mix 2 tablespoons olive oil with salt, black pepper, smoked paprika, garlic powder, and onion powder; set aside.
Heat 1 tablespoon olive oil in a skillet over medium-high heat, then sauté diced onion and garlic for 1 minute.
Add fresh spinach to the skillet and cook until wilted and water evaporates, about 2-3 minutes, then cool slightly.
In a large bowl, combine cream cheese, mozzarella, parmesan, and cooked spinach mixture until evenly mixed.
Stuff each chicken breast with a quarter of the spinach and cheese filling and place in the baking dish.
Brush the tops of the stuffed chicken breasts fully with the prepared olive oil seasoning mixture.
Bake for 35-40 minutes until the internal temperature reaches 165°F, chicken is golden, and filling bubbles.
Rest the chicken for 5-10 minutes before serving to allow filling to set.