Preheat your oven to 190°C (375°F). Lightly grease a 23x33 cm (9x13 inch) baking dish with cooking spray or butter.

In a large skillet or Dutch oven, melt the 50g (1/4 cup) of butter over medium heat. Add the finely chopped 100g (1 cup) of celery and 100g (1 cup) of onion. Sauté for about 5-7 minutes, or until the vegetables are softened and translucent. Be careful not to brown them.

Add the 500ml (2 cups) of chicken broth and the 305g (10.75 oz) can of condensed cream of chicken soup to the skillet with the softened vegetables. Stir well to combine and bring the mixture to a gentle simmer. Cook for about 2-3 minutes, stirring occasionally, until the soup is fully incorporated and the sauce has slightly thickened.

Remove the skillet from the heat. Stir in the 500g (1 lb) of shredded or cubed cooked chicken and the 227g (8 oz) of stuffing mix. Ensure everything is evenly distributed throughout the creamy chicken mixture.

Pour the entire mixture into the prepared 23x33 cm (9x13 inch) baking dish, spreading it evenly. If you prefer a crispier topping, you can reserve a small amount of the stuffing mix (about 1/4 cup) and sprinkle it over the top.

Cover the baking dish tightly with aluminum foil. Bake in the preheated oven at 190°C (375°F) for 20 minutes. This allows the flavors to meld and the casserole to heat through.

Remove the aluminum foil. Continue to bake for another 10-15 minutes, or until the top is golden brown and slightly crispy, and the casserole is bubbling around the edges. If you reserved stuffing mix for the topping, it should be nicely toasted.

Remove the casserole from the oven and let it rest for 5-10 minutes before serving. This allows the casserole to set slightly, making it easier to portion. Garnish with fresh chopped parsley just before serving.