Preheat your oven to 220℃ 200℃ fan 425 ℉ Gas 7
Heat half a tablespoon of olive oil in a large frying pan over medium heat.
Add the chicken cubes and cook for 3–5 minutes, stirring occasionally, until lightly golden and almost cooked through. Season with salt and pepper to taste, then remove from the pan and set aside.
To the same pan add the other half tbsp of oil and the potatoes, onion and mushrooms, season with salt & pepper to taste and fry for about 5 minutes.
When the potatoes soften remove and add to the chicken.
Lower the heat and add the butter and flour to the pan, stirring well until the flour is fully incorporated and a smooth paste forms. Cook gently for another couple of minutes, stirring continuously, to cook out the raw flour taste from the roux.
Slowly pour in the milk, stirring constantly to prevent any lumps from forming. Once fully combined, add the wine and chicken stock. Season with thyme, nutmeg, honey, salt, and pepper, then continue stirring until the sauce is smooth, creamy, and well combined.
Add the cooked chicken and vegetables back into the pan with the sauce and gently stir to combine. Bring the mixture to a gentle boil.
Reduce the heat and allow the filling to simmer for 15–18 minutes, stirring occasionally, until the sauce has thickened and slightly reduced.
Set the mixture aside to cool completely, preferably overnight.
Using the filling while it’s still hot can cause the pastry to soften and become soggy instead of crisp and flaky once baked.
Lightly grease a 25×25 cm baking dish and spoon in the cooled filling. Cover with a sheet of pastry slightly larger than the dish, pressing the edges gently to seal.
Cut a small cross in the centre of the pastry to allow steam to escape while baking.
Brush the pastry with a mixture of 1 beaten egg and 1 tablespoon of cold water.
Bake in a preheated oven for 25–30 minutes, or until the pastry is beautifully golden, crisp, and flaky.