Add the butter to a large frypan over medium-low heat. Add the leeks and sauté for about 10 minutes, until soft and fragrant. Add the mushrooms and cook for a further 5 minutes. Stir through the chicken, stock and mustard until everything is well combined. Remove from heat and stir through the crème fraiche and baby spinach. Season with salt & pepper.
Transfer the mixture to an oven dish or leave in the frypan if oven-proof.
Working one piece of filo pastry at a time, scrunch it with your hand and place on top of the mixture. Repeat until the top of the pie is covered with scrunched filo. Spray the top generously with olive oil spray and place in the oven for 15 – 20 minutes until the pastry is golden and the filling is bubbling.