Slice the peppers into rings, discarding the ends. Set aside.
In a saucepan, combine the water, vinegars, garlic, salt, sweetener, garlic, and red pepper flakes if using. Bring the mixture to a simmer and remove from heat.
Add the peppers to a heat proof jar and pack them down as much as you can, then pour the vinegar mixture over the peppers.
Allow the mixture to cool to room temperature before you secure the lids.
Chill at least 24 hours to allow the peppers to pickle.