Preheat the oven to 200°C.
Melt the butter in a large heavy-based saucepan set over a medium heat. Sweat the mushrooms with a little seasoning until softened, stirring occasionally; usually 6-8 minutes. Remove from the heat and drain away any excess water from the mushrooms.
Meanwhile, roll half the pastry out on a lightly floured surface to 1/2cm thickness. Line a 7 inch ceramic pie plate with the pastry, cutting off the overhanging pastry. Roll the remaining pastry out to a 7 inch round of pastry, roughly 1 cm thick and reserve to one side.
Fill the pastry base with the ham slices, overlapping if necessary. Spoon the mushrooms on top, then arrange the cheese singles on top evenly. Cover with the reserved pastry and seal well at the edges. Brush the top with a beaten egg, then make a small slit in the centre of the pastry to allow any steam to escape while baking.
Bake for 40-45 minutes until the pastry is golden brown in colour and cooked. Remove from the oven and allow to cool a little before turning out and cutting into portions.
Arrange a couple of slices stacked on plates and garnish with the sprig of parsley leaves before serving.
See more Adult Lunchbox Recipes
For top tips on protecting you and your family when preparing raw meat and poultry, visit Food Safety in the Home.