01 - Preheat the oven to 350°F. Grease and line a 9-inch springform cake tin with parchment paper.
02 - Wash the oranges and place them whole, unpeeled, into a saucepan. Cover with cold water, bring to a boil, then simmer for 1 hour. Drain and let cool.
03 - Once cooled, halve the oranges, remove seeds, and blend whole (including peel) into a smooth purée.
04 - In a large bowl, whisk the eggs and caster sugar until pale and fluffy.
05 - Gently fold the orange purée, ground almonds, baking powder, and salt into the egg mixture until just combined.
06 - Pour the batter into the prepared tin and smooth the surface.
07 - Bake for 45 to 50 minutes, or until a skewer inserted in the center comes out clean. If the top browns too quickly, cover loosely with foil.
08 - Allow the cake to cool in the tin for 10 minutes, then transfer to a wire rack. Dust with icing sugar and garnish with orange zest curls or slivered almonds as preferred before serving.