Photograph: Jason Lowe
Put the rhubarb, apple and sugar in a heavy pan, cover and cook on a medium heat for 3-4 minutes, stirring occasionally. Remove the lid and cook for 8-10 minutes over a fairly high heat, until the rhubarb is soft and most of the liquid has evaporated. Remove from the heat and leave to cool.
Preheat the oven to 200°C, fan 180°C, gas 6. To make the pastry, cream the butter and sugar. Sieve the baking powder and flour together, and stir into the butter mix with the salt, then slowly pour in the cream until well mixed. Chill for about 30 minutes before rolling.
Grease a 17-18cm diameter, 2-3cm deep tart tin with butter and roll two-thirds of the pastry to about 2-3mm thick. Line the tin with the pastry and trim the edges. Then line the tin with a circle of greaseproof paper and fill with baking beans. Bake for 15-20 minutes, or until the pastry is lightly coloured.
Meanwhile, roll the remaining pastry out to a circle just a little larger than the tart. If you like, you can cut the pastry into strips about 1cm wide to lay on top in a lattice pattern or just keep as a circle.
Remove the beans and paper from the tart and spoon in the rhubarb mixture. Cover with the pastry top, trim and press on to the edges of the pastry base with your thumb and forefinger. Brush the top with egg and scatter with sugar. Bake for 30-35 minutes or until the top is a golden crisp. Leave to cool to room temperature and serve with thick custard or cream.