Preheat the oven to 180˚C.
In 3 separate bowls, place some flour, 2 eggs beaten, and the breadcrumb season with salt and pepper.
In the centre of each butterflied chicken breast, lay a slice of ham then the cheese. Fold it to close it tightly. Dip the chicken in flour, then the egg mixture then the breadcrumb to coat and seal the ingredients. Sprinkle chicken breasts with salt and pepper.
Place slices of onions on the roasting tray and add the chicken Cordon Bleu.
Roast in the oven for 25-30 minutes depending on the size of the chicken breast. Check for the chicken to be cooked through before removing from the oven!
In the meantime, prepare the sauce. For sauce, in a small saucepan, over medium heat, melt the butter and whisk flour for a minute. Whisk in the milk and keep stirring until boiling and smooth and thickened.
Remove from the heat and stir in cheese until melted. Check the seasoning and keep warm covered until needed.
Once the chicken is cooked remove from the oven and serve immediately with your favourite sides and a drizzle of the cheese sauce.
If using an air fryer, place in the air fryer for 12-15 minutes at 180-200˚C. Check after 12 minutes to see if the chicken is fully cooked through. Close the lid and cook a bit longer if required. Depending on the size of the basket, cook 2 or 4 at the time!