Slice cucumber in half lengthwise and scoop out seeds with a teaspoon. Slice cucumber thinly. Place in a bowl, sprinkle with salt and let stand 30 minutes. Drain, pat dry. Place cucumber, spring onions and radishes in a large bowl.
To make sour cream dressing: whisk sour cream, vinegar, dill, sugar, mustard and pepper until combined. Spoon over cucumber mixture. Toss to coat. Cover and refrigerate 1 hour.