Preheat oven 190C/gas5 Grease and line a 20cm round cake tin, not a loose bottomed one as the juice might run out of the plums. if you only have this kind of tin, wrap a double laye of foil round the outside of the bottom of the tin.
Halve the plums then cut into 1/4s. Put the sugar and lemon in a bowl and stir the plums through to coat. Put them in n overlapping layer in the bottom of the cake tin. If there's any syrup left in the bowl, spoon it over the plums.
Cream the butter and sugar together till nice and fluffy. Add the eggs a bit at a time then fold in the almonds, polenta, baking powder and lemon juice.
Spoon over the plums and smooth the top. Bake for 35-40 mins till golden. Run a knife inside the cake tin then cool for 10 mins before turning out upside down onto a plate.
Nice contrast between the slightly crunchy cake and the soft juicy fruit topping. I like the almonds coming through in the background. It just needs a spoonful of cream for a perfect pudd!