Preheat your oven to 190°C (fan 170°C, gas mark 5).
Heat the butter in a large frying pan over a medium to high heat. Add the leek and mushrooms and fry for 2–3 minutes until they just start to soften. Crank up the heat to high, add the chicken pieces and fry for a further 2 minutes – the chicken won’t be cooked through at this point – then pour in the chicken stock and let it come to a simmer.
Meanwhile, mix the cornflour with 2 tablespoons of water until smooth, then pour into the pan, along with the cream. Bring back to the boil, stirring gently, and cook until the sauce thickens. Remove from the heat and stir in the spinach, then tip the whole lot into a pie dish about 28cm x 15cm. Set aside to cool a little.
Take a sheet of filo and roughly crumple it in your hands – there is no right or wrong to this method! Place the crumpled filo on top of the chicken filling in the pie dish and repeat with the remaining filo sheets.
Drizzle the pastry with olive oil, then bake the pie for about 20 minutes, by which time the filo will have crisped up and turned golden brown in places.
Serve up your pie with fresh salad or some vegetables.