Add the butter/non dairy alternative to a pan with the olive oil, diced onion and sliced mushrooms.
In a bag add the diced chicken thighs, flour and herbs, seal the bag and gently shake until the chicken is thoroughly coated.
Once the onion has started to go translucent, add the chicken and flour and stir. The flour should absorb the fat to make the starts of a roux. Stir until the chicken starts to colour a little.
Add the hot chicken stock to the pan, stiring to create a thick sauce in the pan.
Pour into pie dish and cover with the ready roll puff pastry, trimming the excess and then coating with an egg wash.
Bake in the oven for a good 30-40 minutes until the pastry is golden brown.