Firstly preheat your oven to 180C.
Take each chicken breast and butterfly it so that you can stuff them with your filling.
Put your ham and cheese in the middle and fold the chicken breast shut as tightly as possible. Don't place your filling too close to the outside edge of the chicken breast, or you'll risk it all oozing out when it cooks!
Coat your filled chicken breast in gluten-free flour to give it a nice, light dusting.
Beat your egg in a separate bowl and dredge the filled chicken breast in it, being careful not to lose your filling.
Then coat your egg-washed, stuffed chicken breast in gluten free breadcrumbs. Now you're ready to fry!
Fry your chicken breast on each side until golden brown, then place on a baking tray and place into the oven for 20-25 minutes.
Now it's time to make your sauce! Melt your butter down and then stir in your flour for a couple of minutes. Very gradually pour in your milk, constantly stirring.
Add your cheese and wholegrain mustard. Continue to stir over the heat until it starts to thicken up.
Remove from the heat and stir in a little salt and pepper.
Once your sauce is done, decant it into a little jug ready to serve up with your Cordon Bleu. Enjoy!