Remove the skin from the cucumber, cut in half lengthwise, scrape out the seeds. Set one piece aside for decoration.
Roughly cut the cucumber flesh into cubes. In a frying pan, melt the butter and sauté the shallots until translucent. Add cucumber cubes, season with salt and cook for 15 minutes. Puree, add cream, season with juice of one lemon, pepper and coriander and bring to boil again.
Cut the cucumber into strips and add to the soup. Cool and put in the refrigerator.
Before serving, add the chopped dill and the salmon cut into thin strips.