01 -
In a large bowl, combine the sliced bell peppers and onions.
02 -
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).
03 -
Bring the mixture to a boil over medium heat, stirring until the sugar dissolves.
04 -
Pour the hot vinegar mixture over the sliced peppers and onions. Stir well to combine.
05 -
Let the relish cool to room temperature, then transfer to airtight containers or jars.
06 -
Refrigerate for at least 24 hours before serving to allow the flavors to meld.