Fry the mince in a little oil over a high heat, stirring to break up any lumps, until it’s well browned all over. Transfer the mince to a plate, then fry the onion, carrots and celery in the rest of the oil over a low heat until softened. Stir in the tomato purée and crumble in the stock cube, then return the mince to the pan and give everything a good stir. Fry for a minute, then add 300ml water. Cover with a lid and simmer for 20 mins, then remove the lid and simmer until the sauce has thickened slightly. Stir in the peas, then turn off the heat and leave to cool for 10 mins
Heat oven to 200C/180C fan/gas 6. Divide the mince mixture between four individual pie dishes (or use one large dish). Brush the dish rims with egg. Unroll the pastry and cut it into four pieces for the individual pies – roll the pieces out a little more to fit the pie dishes if you need to. Top each pie with some pastry, press down against the rim and trim any excess. Seal the edges with a fork, or crimp if you like, then brush the pastry with egg.
Poke a little hole in the top, decorate with any offcuts if you like (brushed with a little egg), then cook for 25-30 mins or until the pastry is golden and risen. Cool for 5-10 minutes before serving.