Cook the potatoes in boiling salted water over a low heat, covered, for 5-7 min. Remove the pot from the heat and set aside the potatoes, covered, for a further 5-7 min., or until tender. Drain the potatoes and let them steam off.
Combine the crème fraiche, mustard and salt, cover and refrigerate for at least 15 min. Season to taste.
Cut the cucumber lengthwise and scrape out the seeds with a spoon. Cut the cucumbers in half slices and combine with the warm potatoes and chopped dill. Arrange in 4 deep plates or bowls along with the radishes, spring onions and tenderloin. Pour a little mustard dressing on the salad. Peel the soft-boiled eggs. Break the eggs open or cut them in half and place one egg on top of each portion of salad. Sprinkle with sprigs of dill, salt and pepper and serve the salad with the rest of the dressing and toasted rye bread.