Thinly slice cucumbers and onion. Add cucumbers to a large bowl.
Toss cucumbers with 1 teaspoon kosher salt. Let stand 15 minutes to draw out moisture.
Rinse briefly and drain. Pat dry, then gently squeeze to remove excess liquid.
Whisk sour cream, vinegar, sugar, remaining salt, and black pepper in a small bowl.
Combine cucumbers, onion, and dill. Fold in the dressing until evenly coated.
Chill 10 minutes for flavors to meld. Adjust seasoning to taste and serve.