Preheat oven to 160°C/325°F/Gas Mark 3.
Put the sugar and water into the pan. Stir over a medium heat until the sugar dissolves, then cook without stirring until the sugar caramelizes to a pale golden colour.
Meanwhile cut the rhubarb into 2.5cm (1 inch) pieces, arrange in a single layer over the caramel.
Put the butter, sugar and flour into the bowl of a food processor. Whizz for a second or two, add the eggs and stop as soon as the mixture comes together. Spoon over the rhubarb, spread gently in as even a layer as possible.
Bake in the preheated oven for approximately one hour. The centre should be firm to the touch and the edges slightly shrunk from the sides of the pan. Allow to rest in the pan for 4-5 minutes before turning out. Serve with crème fraiche or softly whipped cream.
