Peel the cucumber, cut in half lengthways and scrape out the seeds with a spoon. Cut the cucumber into 1 cm thin slices, place on a cloth and lightly salt. After 20 minutes, squeeze out the water with the cloth.
Melt the sugar in a hot pan, stir in the butter. Stir the dry cucumber into the caramel and glaze with it. Season with pepper and a little salt, pour in the vinegar.
Cover and simmer the cucumbers over a low heat for 5 minutes. Stir in the dill, creme fraiche and mustard and the cucumbers are ready.