Slice cucumber lengthwise, remove the seeds, and slice thinly.
Thinly slice the onionl.
In a colander, toss the cucumber and onion with the salt and let sit (in the drain) for 20 minutes.
Press the liquid out of the vegetables and rinse well with cold water.
In a mediium bowl combine the vinegar and sugar; stil well.
Add the cucumber mixture and toss to coat. Stir in dill.