Preheat the oven to 180ºC (350ºF, gas mark 4). Line the bottom and sides of a 23 cm (9 in) springform cake tin with baking parchment. Finely chop the oranges in a food processor or blender, or with a large knife. Put the egg whites in a large bowl and whisk until they form stiff peaks. Gradually whisk in half the caster sugar, then whisk for 1 minute. Using the same whisk, whisk the egg yolks with the remaining caster sugar in another bowl for 2–3 minutes or until pale and quite thick. Whisk in the finely chopped oranges, then carefully fold in the ground almonds.