Preheat your oven to 375°F (190°C).
Start by layering your cooked chicken and ham in the bottom of a greased baking dish.
Next, layer slices of Swiss cheese over the chicken and ham.
In a saucepan, melt butter over medium heat. Stir in flour to form a roux, then gradually whisk in chicken broth and heavy cream until smooth and thickened.
Add Dijon mustard, salt, and pepper to the sauce, stirring to combine. Pour the sauce evenly over the chicken, ham, and cheese in the baking dish.
In a small bowl, mix together breadcrumbs with melted butter. Sprinkle this breadcrumb mixture evenly over the top of the casserole.
Bake in the preheated oven for 25-30 minutes, or until the casserole is bubbling and the breadcrumbs are golden brown.
Let it cool for a few minutes before serving, and get ready to dive into comfort food heaven!