Preheat your oven to 200°C (180°C fan). Gather all ingredients before beginning to ensure smooth preparation. Having the pastry chilled while you make the filling helps it stay crisp when baked.

Heat the butter and oil in a large frying pan over medium heat. Add the chopped onion and cook until soft and translucent, about 5 minutes. Stir in the crushed garlic and cook for another minute until fragrant.

Add the sliced mushrooms to the pan and cook for 5–6 minutes until they release their moisture and begin to brown. Season lightly with salt to draw out the water and enhance flavour.

Stir in the chicken pieces and cook until lightly golden on all sides. This step locks in the juices and develops a deeper flavour in the filling.

Sprinkle the plain flour over the mixture and stir well to coat. Cook for one minute to remove the raw flour taste. Gradually pour in the chicken stock while stirring continuously to prevent lumps from forming.

Reduce the heat to low. Stir in the double cream, English mustard, and thyme. Simmer gently for 5–7 minutes until the sauce thickens to a creamy consistency. Adjust seasoning with salt and black pepper. Remove from heat and allow the filling to cool slightly.

Transfer the filling into a pie dish. Roll out the puff pastry on a lightly floured surface and place it over the filling. Trim any excess pastry and crimp the edges to seal. Cut a small slit in the centre to allow steam to escape.

Brush the pastry with beaten egg for a glossy golden finish. Place the pie on a baking tray to catch any drips and bake in the preheated oven for 25–30 minutes until the pastry is puffed and golden brown.

Allow the pie to rest for 5–10 minutes before serving. This helps the filling set slightly, making it easier to cut clean portions.

Serve hot with creamy mashed potatoes or buttered peas. A spoonful of English mustard or a dash of gravy on the side enhances the savoury richness beautifully.