Per Nigel Slater's notes to his recipe, use coarse polenta meal, the one that actually feels a bit gritty. This will create a gritty, sugary crust, which contrasts nicely with the "slithery fruit" which makes him "go back for more." It is best to serve this cake when cool, as it is fragile when warm. But don't forget to serve it together with the reserved juices from the cooked rhubarb.