Toss cucumber and onion slices in a large colander with 2 tsp sea salt. Place the colander over a bowl to catch draining liquid and refrigerate for 1 to 2 hours. The cucumbers will shrink considerably as the salt releases their moisture.
Rinse cucumbers and onions under cold running water to rinse off the salt. Set aside to drain thoroughly, about 20 minutes.
Toss drained cucumber and onion with wine vinegar, dill and a pinch of salt, if desired. Cover and refrigerate 1 to 2 hours more until ready to serve.