Preheat oven to 350° F and lightly grease a 9x13 inch baking dish.
Melt two tbsps of butter in a saucepan over medium heat, add in the all-purpose flour & cook for 1 minute.
Whisk in the milk & Dijon mustard, reduce to a simmer for 3-5 mins or until the sauce thickens.
Turn off heat, then mix in the cheeses.
Melt the remaining two tablespoons of butter in a microwave-safe bowl in the microwave for 30 secs. Add in the breadcrumbs and mix to combine.
Place shredded chicken at the bottom of your casserole dish, then top with the cubed ham.
Pour cheese sauce over the top of the casserole and sprinkle the Panko breadcrumbs on top.
Bake for 25-30 mins or until golden brown and bubbly.