Scoop the seeds out of the cucumber and discard them. Thinly slice the cucumber and sprinkle with salt. Let stand for 30 minutes and drain well. Pat cucumber slices dry.
Combine the cucumber with the spring onion and radish in a small bowl.
To prepare the dressing, whisk the sour cream in a small bowl with the vinegar, dill, sugar, mustard and pepper. Pour the dressing over the cucumber mixture. Toss to coat. Cover and refrigerate for 1 hour.