Put the whole clementine's in a pan, cover with water and boil, covered, for 1 hr until a knife easily pierces them. If the clementine's won’t stay under the water, place a small saucepan lid directly on top to keep them submerged.
Remove the clementine's from the water and cool, then quarter and remove any seeds and obvious pith where the stalk was. Blitz the clementine's to a rough purée with a hand blender or in a food processor and put in a large bowl.
Heat oven to 160C and line the base and sides of a 21cm loose-bottomed cake tin with baking paper.
Mix the ground cardamom, xylitol and eggs thoroughly and stir into the clementine purée.
Mix the almond flour with the polenta, self-raising flour and baking powder.
Fold the flours into the puree and stir until well blended.
Scoop the mixture into the tin, level the top and bake for 40 mins.
After 40 minutes remove the cake carefully from the oven and scatter over the flaked almonds.
Quickly return to the oven and bake for a further 20 mins or until a skewer inserted into the centre comes out clean.
Remove from the tin and leave to cool.
Serve plain with a dusting of icing sugar , or with freshly whipped cream.