Sauce: in a medium-sized pot over medium heat, melt the butter. Whisk in the flour to form a roux, then gradually pour in the milk, whisking as you go to ensure no lumps form. Stir in the seasoning, then bring to a gentle bubble. Turn off the heat and whisk through the cheese until it melts, then taste-test and adjust with salt if needed. Place to one side.
Prep Chicken: slice each breast horizontally right through the centre to create 4 even-sized cutlets.
Prep Dredging Stations: in a small pot or bowl, combine the salt, paprika, onion powder, garlic powder and black pepper, then place to one side. Line up 3 large shallow dishes: 1st with flour and half of the seasoning mix, 2nd with 2 eggs and mustard beaten until smooth, 3rd with Panko and the rest of the seasoning mixed in.
Dredge Chicken: one by one, take a chicken cutlet and coat in the flour, then in the egg, then in the Panko, ensuring you thoroughly coat it at each stage. Place to one side and repeat.
Cook Chicken: add enough oil to comfortably coat the base of a large pan and place it over medium-high heat. You're aiming to get the oil to around 350F/180C - when a breadcrumb rapidly sizzles, it's hot enough. Carefully place in 2 cutlets and fry both sides for 3-4 minutes until deep golden, crisp and white/piping hot through the centre, then place on a wire rack above paper towels (to catch excess oil). Top up the oil if needed and bring back to temperature, then repeat with the last 2 cutlets.
Prep Grill: preheat the grill/broiler. Line a large baking tray with baking paper.
Prep Chicken: at this point, the sauce might be a little thick/lumpy, which realistically is fine, but just gently warm it and give it a thorough whisk to smooth it out. Place the cutlets on the lined tray, then top with ham. Pour the sauce over the cutlets, then sprinkle with parmesan. There will be a lot of sauce; I use it all but you can hold a little back to serve if you'd prefer.
Grill/Broil: place under the grill/broiler until golden and bubbly. Serve up and enjoy!