Preheat the oven to 180°C, gas mark 4.
Butter and line a 20cm-deep, loose-bottomed cake tin.
Cream the butter until soft, then beat in the caster sugar and orange zest, and continue beating until pale and fluffy.
Beat in the eggs a little at a time. Then add the orange juice and mix well.
Fold in the polenta, baking powder and ground almonds to give a soft mixture that drops easily off a wooden spoon.
Spoon half the mixture over the base of the cake tin.
Arrange the rhubarb over the cake base, then drop the rest of the mixture over the top, leaving some gaps. Sprinkle 1 tsp of sugar over rhubarb.
Bake for 1-1¼ hours until well risen and golden and a skewer inserted into the centre emerges clean and dry