To make the Cheese Sauce, melt butter in a pot over a medium heat, add the flour and stir it into the melted butter, cook for about 3 minutes. Slowly add in the milk, continue cooking until mixture thickens, stirring continuously.
Add 150g of the cheese and stir in until melted, season with salt, pepper and mustard.
Allow the Cheese Sauce to cool slightly and then mix in the cooked mushrooms, cooked bacon lardons and chives
Pre-heat oven to 180°C.
Line a baking tray with parchment paper and lay the puff pastry on top.
Use a sharp knife or pizza cutter to cut the puff pastry into 6 squares.
Spoon the cooled filling into the centre of each square, adding more of the cheese on top.
Fold the corners of the puff pastry so that they meet in the centre to create small parcels.
Brush the top of the pastry with the beaten egg and bake until golden, about 20 minutes.