Melt one knob of butter in a large frying pan, add the stuffing balls and cook until crispy. Remove and set aside. If your stuffing has been cooked and is leftover from the day before, there’s no need to fry.
Melt the remaining knob of butter in the pan, stir in the flour, thyme and mustard (if using), and cook for about 1 minute. Add the wine and stir constantly to prevent the sauce from going lumpy.
Add the stock and any remaining gravy, stir well, and bring to a gentle simmer. Stir through the leftover veggies and chicken, then crumble in the stuffing balls. Season with salt and pepper and set aside to cool.
Preheat the oven to 220°C/fan 200°C/gas mark 7. Spoon the filling into the pie dish and brush the edges of the dish with beaten egg before unrolling the pastry and laying it over the top.
Trim away any excess pastry and gently press down on the edges to help create a seal. Cut a steam hole in the centre of the pie and, if you’re feeling creative, use the pastry trimmings to make decorative shapes on top. Brush the pie lid with beaten egg.
Reduce the oven temperature to 200°C/fan 180°C/gas mark 6. Bake in the centre of the oven for about 30 – 35 minutes or until the pastry is golden and crisp.