Heat the butter in a large frying pan over a medium-high heat until it begins to sizzle. Add the chicken pieces and cook until they are golden brown on all sides but not fully cooked.

Add the chopped onion and sliced mushrooms into the pan with the browned chicken pieces and meat juices. Cook for five minutes until the onions are soft and the mushrooms have released their moisture.

Sprinkle the all-purpose flour over the chicken and vegetable mixture and stir it in very well. Cook for two minutes to remove the raw flour taste and create a thick, sticky base.

Slowly pour in the chicken stock while stirring constantly to prevent any lumps from forming in the pan. Bring the mixture to a gentle boil until the sauce begins to look thick and glossy.

Turn the heat down to low and gently stir in the heavy cream and the dried thyme leaves. The sauce will turn a beautiful creamy white color and smell very rich and savory.

Add salt and black pepper to the mixture and taste to see if it needs more seasoning. Pour the hot chicken and mushroom filling into your deep pie dish and spread it out evenly.

Roll out the puff pastry on a lightly floured surface so it is slightly larger than your dish. This ensures the pastry can hang over the edges to create a very secure and pretty seal.

Place the pastry over the filling and press the edges down firmly with a fork to seal. Cut a small slit in the center of the crust to let the hot steam escape.

Use your pastry brush to coat the entire top of the pie with the beaten egg wash. This is the secret to getting that professional, shiny, and golden-brown look after it finishes baking.

Place the pie in a preheated oven at 400°F and bake for twenty to twenty-five minutes. You want the pastry to be puffed up high and the filling to be bubbling hot.