Preheat your oven to 190°C (375°F). Lightly grease a 23cm x 33cm (9x13 inch) baking dish with cooking spray or butter.

In a large bowl, toss the bite-sized chicken pieces with 5g (1 tsp) salt and 2g (1/2 tsp) black pepper until evenly coated. Set aside.

Melt 60g (4 tbsp) butter in a large skillet or Dutch oven over medium heat. Add the finely chopped onion and sauté for about 5-7 minutes, or until softened and translucent. Stir in the minced garlic and cook for another minute until fragrant, being careful not to burn it.

Sprinkle the 60g (1/2 cup) all-purpose flour over the softened onions and garlic. Whisk continuously for 1-2 minutes to cook the flour and create a roux. This helps to thicken the sauce and prevents a raw flour taste.

Gradually whisk in 500ml (2 cups) of chicken broth, a little at a time, ensuring no lumps form. Bring the mixture to a simmer, stirring constantly, until it begins to thicken. This should take about 3-5 minutes.

Reduce the heat to low. Stir in 250ml (1 cup) of heavy cream and 15ml (1 tbsp) of Dijon mustard. Continue to stir until the sauce is smooth and creamy. Let it gently simmer for another 2-3 minutes to meld the flavors, but do not boil.

Add the seasoned chicken pieces and the diced 150g (5.3 oz) cooked ham to the skillet with the sauce. Stir gently to combine, ensuring all chicken is coated. If the sauce seems too thick, you can add a splash more chicken broth or cream.

Stir in 100g (about 3.5 oz) of the shredded Swiss cheese until it is melted and well incorporated into the sauce. Reserve the remaining 100g (about 3.5 oz) for topping.

Pour the chicken and sauce mixture into the prepared baking dish, spreading it evenly. Ensure the chicken pieces are mostly submerged in the sauce.

Evenly sprinkle the reserved 100g (about 3.5 oz) of shredded Swiss cheese over the top of the casserole. Then, distribute the 50g (1/2 cup) of Panko breadcrumbs over the cheese for a crispy topping.

Bake in the preheated oven at 190°C (375°F) for 25-30 minutes, or until the casserole is bubbling around the edges and the topping is golden brown and crisp. If the topping starts to brown too quickly, you can loosely tent the dish with aluminum foil.

Once baked, carefully remove the casserole from the oven. Let it rest for 5-10 minutes before serving. This allows the sauce to set slightly, making it easier to serve. Garnish with chopped fresh parsley, if desired.