Cut the trimmed rhubarb into pieces 15 cm (6 in) long and peel off the stringy bits. Then slice as thinly as possible.
Put the rhubarb into a saucepan with 160g (5½ oz) sugar, seal tightly and let it stew gently for 10 minutes. Take off the lid and put the pan over brisk heat, stirring with a wooden spatula until all the moisture has evaporated and the rhubarb