Combine the cucumber and onion slices.
Sprinkle the salt over the vegetables and let stand for 30 minutes.
Drain well.
Combine the cold vinegar, sugar and mixed pickling spices, stirring until the sugar dissolves.
Pour the vinegar mixture over the cucumber and onion slices.
Transfer to a glass jar and refrigerate.
Allow the pickles to rest for 1–2 days before eating for the best flavor.