Wash the oranges, place them in a large saucepan and cover with water. Bring to boil and simmer for approximately 2 hours. Drain & allow to cool. Coarsley chop the oranges, discarding any seeds (the peel stays on). This step can be done a day in advance & stored in the fridge, if youlike. Forgive me for not including the cooking of the oranges in the preparation time but really, they are just cooking themselves whilst you go about your business.
Place the oranges in the bowl of a food processor and process until smooth. Alternatively, you could use a stick blender or traditional blender.
Whisk the eggs and the sugar, using electric beaters, until thick and pale. Add the orange, almond meal, orange blossom water and baking powder and fold until just combined.
Pour into a greased, 22cm round pan and bake in 170c fan forced for about 50-60 minutes or until skewer inserted comes out clean.
Allow the cake to cool. Dust with icing sugar and garnish with orange zest. Serve with sweetened cream or mascarpone, if desired.