Pickled Pepper and Onion Relish

In a large bowl, combine the sliced bell peppers and onions.
In a medium saucepan, combine the white vinegar, apple cider vinegar, sugar, mustard seeds, celery seeds, turmeric powder, salt, black pepper, and red pepper flakes (if using).
Bring the mixture to a boil over medium heat, stirring until the sugar dissolves.
Pour the hot vinegar mixture over the sliced peppers and onions. Stir well to combine.
Let the relish cool to room temperature, then transfer to airtight containers or jars.
Refrigerate for at least 24 hours before serving to allow the flavors to mel