Preheat your FAN oven to 180 degrees Celsius, 200 degrees Celsius static oven, Gas mark 6 or 400 Fahrenheit.
Peel the potatoes, slice in thin roundels and boil until ¾ cooked, then drain and cool under cold water. Set aside.
Prepare your creamy chicken and mushroom base ingredients: dice the chicken in cubes about 1 inch in size (2.5cm), roughly cut the mushrooms to the same size as the chicken cubes, peel the carrots and cube, clean and dice the celery and onions, as well as the garlic. Wash, lightly dry, and thinly slice the parsley as well.
Prepare your casserole base: In a large pan, add one tablespoon of sunflower oil and warm up on medium heat; once warm add the chicken and cook for about 4-5 minutes, until the chicken changes color on all sides.
Add the chopped garlic to the chicken and cook for a further minute, before adding the mushrooms, onion, carrot, and celery. Mix well, add 200ml of water and allow to cook for 10 minutes, continuously mixing, on a low flame, with a lid on the pan.
Once the carrots are nearly cooked – which they should be after simmering – add the flour and mix well, then add salt, pepper, and parsley and mix well again.
At this point, turn off the heat and add the milk, mix well, and put in the chosen baking dish.
Even out the casserole, without over pressing it, just to make a flat top and lay on top the thin slices of potatoes in a nice design, ensuring all the creamy base is covered. Spray the cooking spray all over the potatoes, then sprinkle with black pepper.
Put in the dish on the middle tray in the oven and bake for 40 minutes, until the potatoes are golden brown and the casserole base is creamy, not watery.
Remove from the oven and allow to sit for about 10 minutes before serving. Enjoy!