Wash, peel and halve the cucumber, scrape out the seeds with a spoon and cut into thin slices. Wash tomatoes and cut into cubes. Peel and finely dice the onions. Mix the cucumber slices, diced tomatoes and onions, sprinkle with salt and let stand for about 20 minutes.
For the sauce, mix the herb mustard with oil, vinegar or white wine and cream. Season to taste and add the chopped dill. Drain the cucumber salad and mix the salad with the sauce.