Preheat the oven to 350 degrees. Using a sharp knife, make slits in the chicken breasts that are about 3/4 inches wide, making sure not to cut all the way down through the bottom of the chicken breast. You will have about 7 or 8 slits per chicken breast, depending on how large your chicken breasts are. Season the chicken with salt and pepper.
Place the spinach evenly in the slits of the chicken.
Combine the breadcrumb and Cajun seasoning in a small bowl. Sprinkle the mixture evenly over the chicken. Place the pepper jack slices in the slits of the chicken.
Place each chicken on its own lightly greased sheet of tin foil and make a "bowl", surrounding each chicken breast. This keeps the juices contained.
Bake in the preheated oven for 30 minutes, or until the chicken is cooked through. Remove each chicken breast from its tin foil "bowl" and place on a serving dish.