Begin by placing the mushrooms in a pan to create a fond at the bottom, which will enhance the flavor of the sauce.
Season the mushrooms with a bit of oil and cook them until they are fully done.
Once cooked, remove the mushrooms from the pan and break them into smaller pieces using your hands, managing the heat to control the moisture released.
Next, add the spring onions and garlic to the pan, cooking them until they are softened.
Incorporate the shredded chicken into the pan and mix it well with the other ingredients.
To make a roux, add a knob of butter and an equal amount of flour to the pan, stirring to combine.
Gradually pour in the chicken stock while whisking the roux until it becomes a smooth velouté sauce.
Allow the sauce to reduce until it thickens to a consistency suitable for pie filling.
Finally, season the filling with salt and stir in cheese to taste.
