1) Preheat the oven to 180 C/Gas 4. Line and lightly butter the base of a 20-cm round non stick cake tin.
2)Separate the eggs. Put the yolks into a bowl with the caster sugar and beat at high speed until they are pale, thick and creamy.
3) Continue to beat, pouring in the lemon juice a little at a time until the mixture starts to thicken.
4) In a separate bowl, mix together the grated lemon zest, polenta and ground almonds then stir in the olive oil. Add this mixture into the beaten eggs and sugar.
5) Beat the egg whites until they are almost stiff then fold into the polenta-almond mixture.
6) Pour the mixture into the prepared cake tin and bake until golden. It is cooked when, after pressing slightly on the top, it springs back up.
7) Meanwhile, make the syrup by combining the caster sugar, water, 3 sprigs of rosemary, slices of lemon and the lemon juice in a saucepan. Heat until it has just come to the boil and the sugar has dissolved, being careful not to let it burn.
8) Remove the cake from the oven, prick the top with a skewer, decorate with lemon slices and pour over the hot syrup.
9) To finish, garnish with a couple of sprigs of rosemary.