Possibly the best dessert in the entire world! Tart, sweet, and with the best crumble topping I have made yet. I usually halve the recipe but I am including the full amount here. It is from Nigella Lawson's cookbook "Feast". It can be made in advance (the crumble topping separate from the rhubarb) and stored in the fridge or freezer. You don't need to thaw the topping before proceeding.