In a small pot, combine the cucumber, bell pepper, onion, and 1/4 cup of the apple cider vinegar. Bring to a simmer and cook for 10 minutes, stirring occasionally. Drain and discard any excess liquid.

Return the veggies to the pot and add the remaining 1/4 cup apple cider vinegar, the sugar, mustard seeds, salt, red pepper flakes, and celery seed. Simmer for 5 minutes.

Remove the relish from the heat and let cool to room temperature. Chill until ready to use.