For the cake, bring a large pot of water to the boil and add the oranges. Simmer the oranges slowly, for approx 2 hours until soft. Drain and allow to cool then remove the seeds and whizz until smooth in a food processor - skin and all.
Preheat the oven to 180 C / 160 C fan. Grease and line a 23 cm springform tin and set aside.
Mix together 250 grams caster sugar with the eggs until pale and fluffy. Add the orange paste then fold in the polenta, ground almonds and baking powder. Pour into the prepared tin and bake for about 1 - 1 ½ hours, until a skewer comes out clean.
Allow to cool completely then decorate with orange zest and serve with some creme fraiche.