About 15 minutes to an hour before serving, toss the cucumber slices in salt and sugar and add to a strainer. This will help the cucumber drain excess water. Pat dry with paper towels.
In a large bowl whisk together the sour cream, milk and vinegar to make the creamy dressing. Add the cucumber slices and stir to combine. Toss with the onion, fresh dill and season with pepper and additional salt if needed.
Serve immediately or cover and chill until ready to serve.