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. 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. Bake for 1-1¼ hours until well risen and golden and a skewer inserted into the centre emerges clean and dry.
150g butter, plus extra for greasing
200g caster sugar
Finely grated zest and juice of 1 large orange
3 Medium Free Range Eggs, beaten
150g polenta
1½ tsp baking powder
75g ground almonds
400g forced rhubarb, trimmed and thickly sliced
1 tbsp granulated sugar