Preheat the oven to 220°C and line a baking sheet with silicone paper.
Mix the vegetables, parsley, seasoning, and enough gravy to bind the mixture together.
Roll one pastry piece to 3mm thick and cut a large disc using a dinner plate.
Score a smaller circle 3cm from the edge, prick with a fork, and place on the tray.
Layer mustard, beef, and vegetables inside the scored circle until ingredients are used.
Brush the pastry border with beaten egg yolk.
Roll the second pastry piece to 5mm thick and drape it over the filling to seal.
Trim excess pastry and use offcuts to create decorative flower shapes on top.
Glaze the entire surface with egg yolk and bake for 20 minutes until golden.
Serve the hot pithivier with additional warm gravy.