Using a mandoline (preferred) or a knife, cut cucumber into very thin slices.
Slice onion slightly thicker–maybe 1/16-inch. Cut round onion slices in half and separate.
Mix brine ingredients stirring until sugar and salt are dissolved.
Place cucumbers and onions in a non-reactive (ceramic, glass or stainless steel) large bowl.
Pour brine over cucumber mixture and stir to make sure it is immersed in the brine.
Cover and refrigerate at least 24 hours.