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