Spread cucumber and onion slices onto a large platter; season with salt on both sides. Let rest for 30 minutes, then drain in a colander and squeeze to remove excess moisture. Blot dry with paper towels.
Stir sour cream, sugar, vinegar, dill, and parsley together in a large bowl. Gently fold in cucumber and onion slices until coated. Cover and refrigerate, 8 hours to overnight.
Garnish with paprika to serve.
