First, make your pastry dough, wrap it in cling film and rest it in the fridge for at least half an hour.
Preheat the oven to 180°C/350°F/gas 4.
Put the butter and sugar into a saucepan and, when the butter has melted, add the apples. Slowly cook for 5 minutes with a lid on, then add the peaches, apricots and plums, stir and cook for 5 more minutes with the lid off.
Meanwhile, remove your pastry from the fridge. Dust your work surface with flour, cut the pastry in half and, using a floured rolling pin, roll one of the pieces out until it’s just under 1cm thick. Butter a shallow 26 cm pie dish and line with the pastry, trimming off any excess round the edges using a sharp knife.
Tip the cooled fruit into a sieve, reserving all the juices, then put the fruit into the lined pie dish so you have a mound in the middle. Spoon over half the reserved juices.
Brush the edge of the pastry with beaten egg. Roll out the second piece of pastry, just as you did the first, and lay it over the top of the pie. Trim the edges as before and crimp them together with your fingers. Brush the top of the pie with the rest of the beaten egg, sprinkle generously with sugar and the cinnamon, and make a couple of slashes in the top of the pastry.
Place the pie on a baking tray and then put it directly on the bottom of the preheated oven for 55 to 60 minutes, until golden brown and crisp. To serve, slice the pie into portions and serve with a generous dollop of half whipped cream.