Chop the pickling cucumbers, yellow onion, red bell pepper, garlic, and jalapeño in a food processor until finely minced.

Transfer the chopped vegetables into a bowl, sprinkle with pickling salt, and soak in ice water for 1-2 hours.

Drain the vegetables using a fine mesh strainer or cheesecloth.

In a pot, combine apple cider vinegar, distilled white vinegar, granulated sugar, mustard seeds, dill, celery seed, turmeric, and crushed red pepper flakes. Heat until boiling.

Add the drained vegetables to the boiling brine and simmer uncovered for about 15 minutes.

Ladle the relish into sterilized mason jars, leaving ½ inch of headspace.

Let the jars cool at room temperature before sealing and refrigerating.