To start, heat the oven to 400* Fahrenheit [180 degrees Celsius (FAN), 200 degrees Celsius static, Gas mark 6].
For the chicken and mushroom casserole, warm up a medium-sized saucepan with some olive oil on medium heat; once warm, add the thinly chopped onions, carrots, celery, and garlic cloves and season with some salt.
Once the onion has softened and turned transparent, add the diced chicken; turn the heat to high and lightly brown the chicken for a couple of minutes. Once all the sides of the chicken have been seared, lower the flame back to medium and cover the saucepan with a lid.
Allow the chicken to cook for 3 to 4 further minutes, then add the chunks of mushrooms; mix well and allow to cook for 5 minutes with a lid on the saucepan, which will prevent the juice of the chicken and mushrooms from evaporating.
The chicken chunks and mushrooms will be nearly cooked, so add your herbs and seasoning of choice (i.e.: parsley and black pepper) and mix well.
Lastly, add the flour to the filling and mix well; once the flour has coated all of the ingredients, add the milk, mix well and remove from the hob.
To prepare the topping , wash and peel two medium-sized white potatoes, then cut them into slices about ⅙ inch [4mm] thick. Bring to the boil a large pot of salted water and add the sliced potatoes to it; allow the potatoes to simmer on a medium flame for about 4 minutes, then drain and allow to lightly cool down, until easy to handle by hand.
Put the hot pot filling in the chosen baking dish (I used a glass dish approximately 8-inches – 21 cm -in diameter) and even the surface out, without overly pressing the content.
Cover the whole surface of the filling using the slices of potatoes, ensuring that each slice is slightly overlapping with the next one.
Cover the dish with a sheet of foil or a lid suitable for baking; put the covered dish on the middle shelf in the oven and bake for 15-20 minutes, then remove the cover and bake for a further 20 minutes, or until the potatoes turn golden brown.
Remove from the oven and allow the dish to rest for 5 to 10 minutes, before serving. Enjoy!