Sift flour and salt into a mixing bowl. Cut butter into flour with a pastry cutter until mixture resembles fine breadcrumbs.
Stir in sugar and add enough water to form a firm dough. Knead lightly then wrap pastry in a plastic bag and chill it in the refrigerator for 30 minutes.
Roll out pastry between two sheets of plastic wrap to fit a 23cm springform pie tin. Peel off top layer of plastic from the pastry then gradually and carefully ease the pastry into the tin. Peel off remaining plastic and trim the edges neatly.
Prick the base with a fork and chill in the refrigerator for 20-30 minutes. Bake blind, covered with foil and filled with beans at 180°C for 15 minutes. Cool on a wire rack.
Separate the eggs. Place the white and yolks in two separate bowls. Add sugar to the egg yolks and beat with an electric whisk until mixture is thick and creamy. Gradually beat in the milk on very low speed. Add flour, lemon and orange juice. In the other bowl, beat the egg white until soft peaks form. Fold in the egg-yolk mixture in two or three batches. Use a plastic spatula to mix, taking care not to stir or beat (this is to prevent deflating the mixture). Do not worry should the mixture look a bit curdled. Pour this lemony mixture straight into the prepared pastry.
Wrap up base of prepared or half-baked pastry crust pan with a piece of foil, taking care not to break the foil. This is done to make sure that water will not seep into the pastry crust during the second time of baking.
Put the pastry pan into another baking dish that is big enough to hold the pan. Add enough boiling water to the baking dish and bake in preheated oven at 180-190°C for about 40-45 minutes or until firm to the touch. Sprinkle lightly with icing sugar if preferred.