Preheat the oven to gas 3, 170c, fan 150c. Grease a 23cm round springform tin and line with nonstick baking paper.
Beat the butter and sugar until pale and creamy. Add the eggs, one at a time, and beat well. Sift in the flour and baking powder, Add the polenta or almonds, and the zest and juice of 1 orange; fold inches.
Spoon into the tin, level the surface. Bake for 40 mins until risen, golden, and a skewer inserted into the cake comes out clean. Remove from the oven but leave in the tin.
Increase the oven temperature to gas 6, 200c, fan 180c. Arrange the rhubarb on a baking tray. Pour over the remaining Orange juice, Add the sugar, and toss to coat. Roast for 10 mins until just tender. Cool on the tray for 5 minutes.
Turn out the cake. Top with rhubarb, spoon the juices over, and scatter with pistachios. Serve with creme fraiche, if you like.