Wash the cucumbers well and remove the stems.
Peel the cucumbers, cut them in half lengthwise, and scrape out the seeds.
Cut the cucumbers into large pieces, then pulse them in a food processor until they reach a small relish-style texture.
Remove the seeds from the peppers and chop them in the food processor.
Peel and quarter the onions, then pulse them in the food processor until finely chopped.
Combine the cucumbers, peppers, and onions in a large pot.
Cover the vegetables with pickling salt, stir well, cover the pot, and let the mixture sit overnight.
The next day, drain the liquid from the vegetable mixture.
Return the drained vegetables to a large pot.
Add the vinegar, sugar, celery seed, mustard seed, and turmeric if using.
Bring the mixture to a boil, stirring from the bottom so the sugar dissolves evenly.
Reduce the heat and simmer for 30 minutes, stirring occasionally, until the relish is softened and the liquid looks lightly syrupy.
Ladle the hot relish into pint or half-pint canning jars, leaving proper headspace.
Remove air bubbles, wipe the rims clean, and place the lids and rings on finger tight.
Process the jars in a water bath canner for 10 minutes, adjusting the time for your altitude.
Let the jars rest undisturbed until fully cooled, then check the seals before storing.
