01 -
Grab a large mixing bowl and toss in your thinly sliced bell peppers and onions. Mix them up so they're evenly combined.
02 -
In a medium saucepan, pour in both vinegars and add the sugar, mustard seeds, celery seeds, turmeric, salt, black pepper, and red pepper flakes if you're using them.
03 -
Set your saucepan over medium heat and bring everything to a gentle boil. Stir occasionally until you can't see any sugar granules – they should all be dissolved.
04 -
Carefully pour the hot vinegar mixture over your peppers and onions in the bowl. Give everything a good stir to make sure all the vegetables are coated in the pickling liquid.
05 -
Let your relish cool down to room temperature before transferring it to clean jars or containers with tight-fitting lids.
06 -
Pop your relish in the refrigerator and try to wait at least 24 hours before digging in. This resting time lets all those flavors mingle and develop.