Cut cucumbers in half lengthwise and scoop out the seeds.

Chop cucumbers, onions, green bell pepper, and red bell pepper.

Place all chopped vegetables in a large bowl.

Sprinkle kosher salt over the vegetables.

Cover with cold water and let stand for 2 hours.

Drain vegetables well and press out as much liquid as possible.

In a large pot, combine sugar, cider vinegar, celery seed, and mustard seeds.

Bring to a boil.

Add the drained vegetables to the boiling mixture.

Bring back to a boil and simmer for 10 minutes.

Using a slotted spoon, transfer the relish into pint jars, following standard canning procedures.

Ensure some brine is added to each jar, but avoid making it too soupy.

Process in a hot water bath according to your altitude (10 minutes for up 50 1000 ft.).