Pour the stock into the slow cooker and turn to High. Mix together the Worcestershire sauce, tomato purée and cornflour, then whisk into the stock.

Heat half the oil in a large pan and fry the beef over a high heat until browned. It's best to do this in two batches, then transfer to the slow cooker. Add the remaining oil to the pan, lower the heat slightly and fry the onions, carrots and celery with some seasoning for 10 mins until golden. Add to the slow cooker along with the thyme or rosemary. Cook on Low for 8 hrs or High for 4 hrs until the beef is very tender. Leave to cool then keep chilled until ready to use. Will keep chilled overnight.

To make the flaky pastry, put the flour, 1/2 tsp fine salt and the butter in a large bowl. Add 115ml very cold water and, using a cutlery knife, bring together into a rough dough. Cover and chill for 30 mins.

On a lightly floured work surface, roll the dough out into a 1cm-thick rectangle that is three times as long as it is wide. Fold the top third of the pastry down and the bottom third up over the top to create a block. Turn 90 degrees and roll out to the same size rectangle as before, then fold in the same way again. Cover and chill for 30 mins.

Repeat the rolling and folding four times, then wrap and chill for at least 30 mins until ready to use.

Heat the oven to 200C/180C fan/gas 6. Spoon the pie filling into an ovenproof dish, then roll the pastry out until large enough to cover the dish. Brush the rim with some of the egg, then lay the pastry over the top. Trim the overhanging pastry, then crimp the edges. Brush with the remaining egg, make a couple of slits for the air to escape, then put the dish on a baking sheet and bake for 40-45 mins until puffed and dark golden. Leave to cool a little before serving.