Begin by coarsely chopping the cucumbers, sweet peppers, and onions. Using a food processor, pulse the vegetables until finely chopped but not pureed.

In a large bowl, combine finely chopped vegetables and sprinkle evenly with kosher salt. Pour in enough ice water to cover and stir gently. Let sit undisturbed for 2 hours.

After 2 hours, drain the vegetable mixture thoroughly. Rinse under cold water to remove excess salt and squeeze out remaining moisture.

In a large pot, combine sugar, cider vinegar, mustard seed, and celery seed. Over medium-high heat, bring to a boil, stirring until the sugar dissolves.

Once the brine reaches a rolling boil, stir in the drained vegetable mixture. Reduce heat to low and let it simmer for about 10 minutes.

Using a ladle, spoon the hot relish into prepped sterilized pint jars, leaving about ¼ inch of headspace at the top. Wipe the rims of the jars to ensure proper sealing.

Seal the jars with lids and process them in a boiling water bath for 15 minutes.

Once processed, remove jars from boiling water and allow to cool completely at room temperature.