Melt butter in a medium pan over a low heat and fry the leeks until soft, then add mushrooms and continue to fry until golden.

Stir in flour and cook for 1 minute, then gradually add the stock, stirring constantly until the sauce is smooth. Reduce the temperature and simmer for 10 minutes until thick.

Stir in double cream, mustard, cooked chicken, spinach, cheese, chives and season.

Preheat the oven.

Unroll the filo pastry and cover with a clean, damp tea-towel.

Lay a single sheet of filo onto a board and brush with a little melted butter. Roughly scrunch up the pastry and arrange on top of the pie, repeat until the top of the pie is covered.

Bake for 20 – 25 minutes until the filo pastry is golden.