Begin by thinly slicing your pickling cucumbers and white onion. In a large mixing bowl, combine the sliced cucumbers and onion with kosher salt and let them rest in the fridge for 1 hour.

Rinse the cucumbers and onions in a colander under cold water and drain them thoroughly for about 10 minutes.

In a medium saucepan, combine light brown sugar, white vinegar, apple cider vinegar, celery seeds, mustard seeds, and optional red pepper flakes. Stir until the sugar dissolves and the mixture is clear.

Remove the saucepan from heat, let it cool for 5 minutes, and then pour the warm brine over the rinsed cucumbers and onions in a large glass jar.

Allow the jar to cool to room temperature for about 30 minutes, then seal and refrigerate. Let the pickles marinate for at least 24 hours.

Enjoy your pickles cold as a side dish or on sandwiches!