Preheat the oven to Gas Mark 7, 220°C, fan 200°C.
Place the flour in a bowl, then add the butter and rub it into the flour until it resembles breadcrumbs. Stir in 1 tablespoon of the sugar plus the cinnamon. Then add 3 tablespoons of cold water and mix to a soft dough. Turn out onto a well-floured work surface and shape into a ball. Cut into 6 equal pieces. Roll out each piece into a circle about 14cm (approximately 6in).
Cut the plums in half, remove the stones and cut each half into 3 slices. Place in a bowl and toss with the remainder of sugar, the walnuts and orange rind. Divide the filling between each pastry circle, piling it into the centre. Gather 1.5cm (¾in) of the pastry edge up over the plum filling, pinching it around to make a rim. Brush the pastry edge with egg white. Sprinkle with a little extra sugar.
Bake for 15 minutes until the pastry is golden brown and crisp. Serve warm with cream or custard.