If using large cucumbers, cut in half lengthwise and scoop out seeds with a spoon.
Combine the diced cucumbers, onions and peppers in a large stock pot. Sprinkle the 2 TBSP kosher salt over the vegetable. Fill the pot with ice cold water enough to cover the diced vegetables. Set the mixture aside to rest for 2 hours.
Line a fine mesh sieve with cheesecloth and add the vegetables. Allow them to drain for 10 minutes, then gather the ends of the cheesecloth together and twist to form a bundle. Squeeze a little tighter to press out the liquid, allowing it to drain. Don’t smash the vegetables but gently extract as much liquid as possible.
While the vegetables are draining, combine the vinegar, sugar, celery and mustard seeds in the large pot.
Bring the mixture to a boil and add the vegetables.
Stir and bring it back to a boil. Reduce the heat and simmer for 10 minutes.
Using a slotted spoon, place the hot relish in glass jars with lids. Spoon additional brine over the vegetable if needed to cover. If preserving, process according to standard canning procedures.
If not canning, cool the jars then store the jars of relish in the refrigerator. This relish will keep for several weeks if refrigerated.