Using a sharp knife or mandolin, slice cucumber into thin slices.
Slice onion in half, then thinly slice to create half rings.
Add onions and cucumbers to bowl with dill. Add vinegar, sugar, salt and pepper.
Cover with plastic wrap, then use another mixing bowl pressed into the covered salad to add pressure (I like to place something in the bowl to add weight). Refrigerate for at least 30 minutes (the longer the better) to overnight.