Combine sliced cucumber, medium red onion, white vinegar, and Kosher salt in a medium bow, then toss well. Allow this to sit.
In a separate large bowl, whisk together sour cream, mayo, fresh chopped dill, lemon zest, fresh lemon juice, Kosher salt, and freshly ground black pepper.
Add the drained cucumber to the dressing, then stir to evenly coat. Refrigerate for at least 1 hour before serving. Taste, adjust seasoning if needed, then top with additional fresh dill, if desired. Enjoy!