Preheat the oven to 180°C(Fan:170°C)/350°F/Gas 4.
Grease and line the bottom and sides of a 23cm springform tin with greaseproof paper.
Place the oranges in a saucepan with 15ml (one tablespoon) of water. Cover and cook for 30 minutes until soft.
Drain the cooking liquid and spoon the oranges into a food processor. Turn the processor on, and pulse the oranges until they are pulped.
In a spotlessly clean bowl, whisk the egg whites until they form stiff peaks. Whisk half of the caster sugar into the egg whites and mix until the mixture is glossy.
Whisk the egg yolks with the remaining sugar until thick and pale lemon in colour.
Mix in the oranges, and then fold in the ground almonds.
Stir a large spoonful of the egg white mixture into the yolk mixture, then fold in the remaining whites. Transfer to the tin and sprinkle over the flaked almonds.
Bake for 50-55 minutes until a knife or skewer inserted into the centre comes out clean. Leave to cool for 10 minutes, and then turn out onto a wire tray to cool fully. Dust with icing sugar to serve.