Dry-fry onion, carrots, celery, and beef in a large non-stick saucepan over high heat for 3 minutes.
Lower heat and add ketchup, brown sauce, Worcestershire sauce, flour, chilli, and pepper.
Add potatoes and beef stock to the pan.
Simmer loosely covered for 25 minutes until potatoes are tender and mixture is thick.
Stir in frozen peas and leave the mixture to cool completely.
Mix pizza base packets with lukewarm water in a large bowl according to instructions.
Knead dough on a floured surface for 5 minutes until smooth and elastic.
Preheat the oven to 180°C.
Divide the dough into six 80g portions.
Remove 35g from each portion to use as pie lids.
Roll larger dough pieces to 3mm thickness to line the foil pie dishes.
Press dough into the bases and sides of the dishes, leaving some overhang.
Fill each case with the beef mixture and brush edges with milk.
Roll out smaller portions, place over filling, pinch edges, and trim with scissors.
Arrange all six pies onto a baking tray.
Snip a steam hole in the centre of each lid and brush tops with milk.
Bake for 15 minutes, then cover each pie tightly with foil.
Bake for 15 more minutes until golden brown and filling is hot.
Let pies stand for 5 minutes before removing foil and serving.