Heat butter in your large pan.
Add diced chicken pieces to the pan.
Season with salt and pepper generously.
Cook for 5-7 minutes until golden.
Remove chicken and set it aside.
Add mushrooms to the same pan.
Cook them for 4-5 minutes.
They should release their moisture completely.
Add onion and garlic next.
Stir everything for 2 minutes.
Sprinkle flour over the mushroom mixture.
Stir well for one minute.
Slowly pour in the chicken stock.
Add cream and fresh thyme leaves.
Let it simmer for 5 minutes.
The sauce should thicken nicely.
Return the cooked chicken to pan.
Mix everything together gently.
Let it cool for 10 minutes.
This prevents soggy pastry later.
Pour filling into your pie dish.
Roll out the puff pastry sheet.
Cover the dish with pastry carefully.
Trim excess pastry from the edges.
Press edges to seal them tight.
Brush beaten egg over the top.
Preheat your oven to 200°C.
Bake the pie for 25-30 minutes.
The pastry should turn golden brown.
Let it rest 5 minutes before serving.