In a large bowl, put chopped cucumbers, onion, and bell peppers in and sprinkle salt on top.
Cover with cold water and let sit for 2 hours.
Drain well and squeeze out excess water from vegetables. Set aside.
In a large stove pot, add sugar, apple cider vinegar, celery seed and mustard seed over medium-high heat. Bring to a boil.
Add vegetables and bring back to a boil. Reduce heat to medium-low, cover and let simmer for 10-15 minutes.
Use a slotted spoon to place in pint-sized mason jar (follow proper canning procedures if making in bulk). Add just enough of the liquid (brine) to cover the vegetables but there will be leftover brine.
Prep time includes soaking time.*Pictures are more of a chunky relish, which is achieved by roughly chopping vegetables instead of dicing.*Calories include all of the sugar in the brine, but not all will actually be absorbed by the vegetables.*Recipe adapted from Genius Kitchen