Trim the puff pastry sheet to fit your pie dish, and reserve the cut-off pastry.
With these, shape into thin strips to go round your dish.
Brush a little beaten egg around the rim and press the strips down onto it, these will help the pastry lid to stick.
Give the pastry strips a brush with beaten egg, lay the puff pastry sheet on top and crimp the edges to give a good seal.
If you have any pastry scraps left, roll them out and make some decorations for the pie lid. A few leaves, or letters or whatever you fancy, but nothing too personal if your mother-in-law is at the table!
Egg wash the pastry lid, stick on your decoration, then egg wash those. Or sprinkle the pie lid with some freshly grated parmesan, for an extra savoury hit.