Melt 15g butter with oil in a pan and soften onion, celery, carrots, and potatoes for 15 minutes.
Stir in ketchup and chunky pieces of corned beef.
Season with pepper and cool for 20 minutes.
Preheat the oven to 190°C.
Pulse flour, salt, and 175g butter in a food processor until it looks like crumbs.
Mix the egg with water and blend into the flour until a dough ball forms.
Line a 23cm dish with one-third of the rolled pastry and brush the edge with egg.
Fill the base with beef mixture and cover with the remaining rolled pastry.
Seal the edges, brush with egg, and bake on a tray for 40 minutes until golden-brown.