Heat oven to 160C/fan 180C/gas 4. Boil the kettle and thinly slice 5 of the clementines horizontally, discarding the ends. In a small pan, melt 250g
of the sugar with 300ml boiling water, then increase the heat until the syrup is simmering. Add the clementine slices, then cover and cook for 20 mins until the slices are tender. Meanwhile, grease and line the base of a 25cm springform tin.
Use a slotted spoon to lift out the clementine slices, then arrange over the bottom of the tin. Grate the zest from the remaining 7 clementines and set aside. Squeeze the juice from 4, then stir this into the syrup. Boil for 10 mins until thick. Cool and set aside.
For the cake, beat 200g of the remaining sugar with the butter, lemon zest and half of the clementine zest until pale. Beat in the egg yolks one by one. Peel the three remaining zested clementines, then break the segments into a food processor and whizz until pulpy. Stir this into the butter mixture along with the almonds, polenta and one pot of yogurt. Whisk the egg whites to stiff peaks, then gently fold into the cake mix. Carefully spoon the mixture into the prepared tin, then bake for 1 hr until a skewer inserted comes out clean. Cool the cake in the tin.
For the cheesecake cream, mix the remaining clementine zest with the mascarpone, the remaining pot of yogurt, 50g sugar and a good squeeze of lemon juice. Turn the cake out upside down onto a serving plate , then pour over some of the sticky syrup. Serve in wedges topped with
a spoonful of the cheesecake cream, and a little more syrup on the side.