Mix together the Cajun seasoning ingredients in a bowl and set aside.
Place the chicken breasts on a chopping board. Lay a piece of greaseproof paper over and gently bash the fat ends with a rolling pin so the breast is the same thickness all along.
Cut each flattened chicken breast into two.
Coat the chicken pieces in the Cajun seasoning on both sides.
Place a large frying pan over a medium heat and spray with low calorie cooking spray. Place the chicken pieces in the pan and cook on one side for 3 minutes.
Spray the top of the chicken with more low-calorie cooking spray and flip. Add the cheese slice to the top cooking for 3 minutes on the other side. The chicken should be white throughout and the cheese melted.
Take a bun (you can lightly toast if you like) and place one of the chicken pieces inside with some spinach leaves. Repeat with the rest of the buns and serve with an accompaniment of your choice.