Toss together cucumbers, salt, and sugar in a medium bowl. Set aside, and let stand 10 minutes.
If substituting red onions for shallot, soak in ice water in a small bowl & rest.
Meanwhile, stir together shallot and vinegar in a small bowl; let stand 10 minutes.
Add sour cream, dill, black pepper, and shallot mixture to bowl with cucumbers; stir until well combined.
Season with additional salt to taste.
Transfer to a serving bowl.
Garnish with additional dill, & black pepper.Serve immediately.
