Peel, wash and dice the potatoes and boil them in salted water.
Peel the cucumber, cut in half, scrape out the seeds with a spoon, cut the cucumber into thick slices.
Braise the cucumber slices in 30 g butter. Dust the flour over it, stir in, sweat lightly. Pour in the stock, bring to the boil and then cook covered for 15-20 minutes. Add the cream and potato cubes and season with mustard, salt and pepper.
Heat the rest of the butter and fry the mustard seeds vigorously. (Be careful, they like to jump out of the pan!) Chop the dill. Scatter mustard seeds and dill over the vegetables.