Start by separating the eggs. Crack each egg and carefully separate the yolks from the whites. Place the whites in a clean, dry mixing bowl and set them aside for later.

Using a hand mixer or a whisk, beat the egg whites until they form stiff peaks. This means the whites will hold their shape when you lift the beaters out. Set the beaten egg whites aside.

In a saucepan over medium heat, combine the milk, flour, breadcrumbs, butter, pepper, and salt. Stir continuously until the mixture thickens slightly, which should take about 2 minutes. Remove the saucepan from the heat once it reaches the desired consistency.

Add the cooked and cut turkey, egg yolks, chopped onions, chopped celery, and parsley flakes to the saucepan. Mix everything together thoroughly until well combined.

Carefully fold the beaten egg whites into the turkey mixture. Use a gentle folding motion to ensure the egg whites are evenly incorporated without deflating them too much.

Grease a 2-quart casserole dish with butter to prevent sticking. Pour the turkey mixture into the prepared dish, spreading it out evenly.

Preheat your oven to 350°F (175°C). Place the casserole dish in the preheated oven and bake the soufflé for approximately 35 minutes, or until it is puffed up and golden brown on top.

While the soufflé is baking, prepare the mushroom sauce by combining the creamy mushroom soup and water in a saucepan. Heat the mixture over medium heat until warmed through, stirring occasionally.

Once the soufflé is done baking, remove it from the oven and allow it to cool for a few minutes. Serve the turkey soufflé hot, with the warm mushroom sauce drizzled over the top.