Preheat your oven to 450°F.
Cut a pocket into the side of each chicken breast, being careful not to cut all the way through to the other side.
In a large bowl, mix the spinach, cream cheese, mozzarella, and parmesan until evenly combined.
Stuff each chicken breast with the spinach mixture until about 90% full, allowing enough space to close the pocket. Secure the opening of the pocket with toothpicks to prevent spilling.
Place chicken breasts in a single layer on a large baking sheet. Brush both sides of the chicken breasts with olive oil and season with salt and pepper.
Bake for 18-20 minutes, or until the internal temperature of the thickest part of the breast reaches 165°F.
Once the chicken is done, transfer the chicken to a plate and loosely cover it with foil. Allow to rest for 5 minutes.