Preheat the oven to 200ºC, fan 180ºC, gas 6. Steam or microwave the casserole vegetables according to pack directions. Transfer to a large pan along with the leftover cooked veg, the cooking sauce and the leftover roast beef. Simmer over a low heat for 2-3 minutes, stirring,,then transfer to a 1.5 litre pie dish.
Unroll the pastry and place over the top of the dish. Trim the overhanging pastry with a sharp knife, then crimp the edge of the pastry using your forefinger and thumb to seal. Cut a small cross in the centre of the pastry as a vent. Decorate the top of the pie with leaf shapes made from the trimmings, if you like. Brush the top with the beaten egg.
Bake on the top shelf of the oven for 20-25 minutes until the pastry is golden. Five minutes before the end of cooking time, boil or steam the beans and serve with the pie.