Prep cucumbers: Place cucumber in a bowl and cover with water. Stir in 1.5 teaspoons salt. Chill for 1-2 hours. Drain in a colander and rinse under fresh water. Drain well and pat dry with a paper towel.
Make dressing: Whisk together sour cream, dill, vinegar, sugar, remaining salt and black pepper in a salad bowl. Add cucumbers and onion. Toss well and serve cold.