For the custard filling:
Preheat the oven to 180C. For the pastry put the butter, flour and sugar into a food processor and blitz until it resembles breadcrumbs. Add the egg and blitz until a smooth dough forms. You can also do this by hand. Flatten into a disc and cover with clingfilm. Leave to rest in the fridge for 30 mins.
Place the rhubarb into an ovenproof dish. Pour over the sugar. Cover with foil and bake for 25 minutes. Roll the pastry out on a floured surface. Line a loose-bottomed 12-inch tin with the pastry. It’s fine if the edges are messy. Line the inside of the pastry shell with tin foil, press the foil against the pastry so it stays in place. Bake blind for 10-12 minutes. Remove the tin foil and brush inside the entire pastry shell with egg white. Bake for a further 10-12 minutes until completely golden. Leave to cool slightly then remove from the tin. Using a sharp knife trim the edges so it is neat and even.
To make the custard heat the milk until almost boiling. Mix the sugar and custard powder with a little milk until it forms a smooth paste, keep whisking in the milk until it’s smooth. Return to the saucepan and bring to the boil, whisking all the time. Remove from the heat and cool quickly in an ice basin. Place clingfilm on top of the custard. Once the custard is cool swirl it through the whipped cream. You may need to sieve it if there are any lumps.
Place the rhubarb pieces into the pastry case and top with the custard cream mix. Serve right away.