Bring a pan of water to the boil. Lower the orange in and cover with a lid, leaving a gap so that a little steam can still escape. Boil gently for 30 mins, then remove from the water and allow to cool slightly. Quarter the orange and remove any pips, then transfer to a blender or food processor, add the lemon juice and zest and blitz to a smooth purée.
Heat the oven to 180C/160C fan/ gas 4. Oil the sides of a 23cm round cake tin and line the bottom of the tin with baking parchment.
Sieve the almonds, polenta, baking powder and ½ tsp salt into a large bowl and mix together thoroughly. Whisk the eggs and sugar in the bowl of a stand mixer until really pale and fluffy, about 4 mins. With the mixer still whisking, slowly pour in the olive oil in a steady stream so that the mixture doesn’t split. Once incorporated, scrape in the orange purée and whisk to combine, then add the dry ingredients, combining until you have a smooth batter. Pour the batter into the cake tin and bake for 55 mins to 1 hr 5 mins until a skewer inserted into the middle comes out clean.
Cool in the tin on a wire rack while you make the syrup. Combine the sugar, orange juice, lemon juice, cardamom and 30ml water in a pan. Bring to the boil, stirring until the sugar has dissolved, then add the rosemary. Turn off the heat and infuse for at least 30 mins.
Use a skewer to poke holes in the top of the cake, then pour over the syrup. Allow to settle for 5 mins or so, then slice and serve with some natural yogurt or crème fraîche, if you like.