Slice the cucumbers and onions thinly. Place them in a large bowl.
Sprinkle the salt over the cucumbers and onions. Toss to combine. Let stand for 2 hours, stirring occasionally.
Drain the cucumbers and onions thoroughly. Rinse them under cold water and drain again, pressing out as much liquid as possible.
In a large saucepan, combine the sugar, vinegar, mustard seed, celery seed, turmeric, and black pepper. Bring to a boil over medium-high heat, stirring until the sugar is dissolved.
Add the drained cucumbers and onions to the saucepan. Reduce heat and simmer for 10 minutes, stirring occasionally.
Spoon the pickles and brine into sterilized jars. Let cool completely before sealing and refrigerating.
Notes
Best enjoyed after chilling in the refrigerator for at least 24 hours to allow the flavors to meld.