Preheat oven to 190 degrees C.
In a medium bowl, mix mayonnaise, spinach, feta cheese, and garlic until well blended. Set aside.
Carefully chop chicken breasts, making sure not to cut all the way through. Spoon spinach mixture into chicken breasts.
Wrap each with a slice of turkey. Place in a covered baking dish.
Bake in the preheated oven for 1 hour, or until chicken is no longer pink in the center.