Preheat the oven to 180°C.
Begin by using a sharp knife and make a pocket lengthways inside each chicken breast (don’t cut all the way through). Use your finger to widen and open the hole to allow for filling. Place a the ham in centre of one cheese slice, roll up cheese and place into a breast, pushing deep into pocket. Seal opening with toothpicks. Repeat this process for remaining chicken breasts.
Cover the filled chicken breast with plastic wrap and put them in the fridge until firm, at least 20 minutes.
Meanwhile whisk in a medium size mixing bowl the egg whites and mustard together until combined.
Pat the chicken breasts dry with paper towels and season with the
salt and pepper. Dredge the chicken in the flour and shake off the excess.
Place breadcrumbs on a large dinner plate, dip each breast into egg mix, and coat with breadcrumbs. Place on a baking tray that has been generously coated with vegetable oil spray, spray tops of chicken. Bake 20-25 minutes or until chicken is cooked through.
Let the chicken rest for 5 minutes before serving.