Preheat the oven to 220C/200C Fan/Gas 7. Line a large baking sheet with silicone baking paper.
Mix the vegetables and parsley together in a bowl, then season, to taste, with salt and freshly ground black pepper. Squashing the mixture slightly into a mass, adding enough of the leftover gravy for it to stick together.
Roll out one piece of pastry onto a lightly floured work surface to a thickness of 3mm. Upturn a dinner plate onto the pastry and cut round it using a sharp knife to create a large pastry disc. Find a bowl that is about 3cm/1¼in smaller in diameter than the pastry disc, then upturn it in the centre and use it as a template to score a smaller circle on the pastry. Transfer the pastry disc to the prepared baking tray.
Prick the inner pastry circle several times with a fork. Spread over some of the mustard, then arrange some of the cooked beef on top, then spread some of the vegetable mixture on top of that, always keeping inside the scored circle.
Spoon alternate layers of the mustard, beef, and vegetable mixture in the centre of the pastry disc until everything is used up.
Brush the pastry border with some of the beaten egg.
Roll out the second piece of pastry onto a lightly floured work surface to a thickness of about 5mm/¼in. Lay the pastry over the top of the dome of filling, pressing down gently around the edges to seal the two pieces of pastry together.
Trim away any excess pastry. Use any offcuts to decorate the top of the pithivier with a flower shape. Use a butter knife to score details into the petals and stem, if desired.
Brush the surface of the pithivier all over with the remaining beaten egg. Bake in the oven for 20 minutes, or until the filling is piping hot and the pastry is crisp and golden-brown.
Serve the pithivier with the remaining leftover gravy.