Open the jar of salad potatoes, empty into a large bowl. Dump any water off into the sink. Cover with fresh water from tap, and wash any starch residue off.
Set aside to let drain well through a sieve for a few minutes.
Wash, peel and chop the onion.
Mix all the ingredients from the prepared onion down to and including the salt, if using.
Mix the potatoes and dressing together. Chill for 2 hours, more is even better.
Serve sprinkled with the bacon bits and parsley, if using.