Wash and scrub the cucumbers under cold running water, trim off both ends, leave skin on. Chop into large chunks, then finely chop in a food processor (use pulse) or put through a food mill. Put in a large pot or very large bowl. Sprinkle with the salt OR salt sub, mix the vegetables and salt with your hands. Top with enough cold water to cover completely. (The exact quantity of water doesn't matter.) Let stand for 2 hours.
Meanwhile, you can prep the onion and set aside.
Drain the cucumber, rinse and drain again.
In large pot, mix together everything from the vinegar down to and including the mustard seed, then stir in the onion and then the cucumber.
Bring to a boil, lower to a simmer and simmer for 10 minutes.
Pack hot into quarter-litre (½ US pint) jars or half-litre (US pint) jars.
Leave 2 cm (½ inch) headspace for either size jar.
Debubble, adjust headspace.
Wipe jar rims.
Put lids on.
Process in a water bath or steam canner.
Process either size jar for 15 minutes; increase time as needed for your altitude.