In a large bowl or stock pot, combine the chopped cucumbers, bell peppers, and onions. Cover the vegetables with water and stir in the salt until dissolved. Refrigerate for several hours or overnight to draw out excess moisture.

Drain the vegetables thoroughly in a colander. Rinse well with cold water to remove excess salt, then squeeze or press out as much liquid as possible. Removing excess moisture helps keep the relish crisp and flavorful.

In a large stock pot, combine the apple cider vinegar, sugar, honey, mustard seed, and celery seed. Bring the mixture to a gentle boil over medium heat, stirring until the sugar and honey are fully dissolved.

Add the drained vegetables to the brine mixture and stir well to combine. Simmer for 10–15 minutes, stirring occasionally, until the vegetables soften slightly and the flavors blend together.

While the relish cooks, wash and sanitize mason jars, lids, and bands according to safe canning practices. Keep the jars warm until ready to fill.

Using a funnel and ladle, carefully fill the jars with the hot relish, leaving about 1/2 inch of headspace. Remove air bubbles and wipe the jar rims clean before applying lids and bands fingertip tight.

Place the filled jars into a boiling water bath canner, ensuring the jars are fully covered with water. Process for 10 minutes, adjusting for altitude if necessary.

Carefully remove the jars and place them on a towel-lined surface to cool completely. Check that the lids have sealed properly before storing. Store sealed jars in a cool, dark place. Refrigerate after opening.