Pack sliced onions firmly into 12 ounce glass jar. Layer in any herbs you want.
Dissolve salt in 12 ounces of filtered water and add whey.
Pour brine over onions and tilt jar a few times to mix well. Leave one inch between the top of the jar and the top of the brine. Use a weight (find a good source for fermenting weights here) to keep the onions under the brine, if necessary.
Let sit on counter for three to seven days. Use an airlock lid (find a good source for airlock lids here) or burp the jars (meaning just open them) every day to let out the gasses that are a natural byproduct of fermentation. If the water level rises, simply pour some out.
Enjoy on salads, burgers or anywhere you usually use red onions. Fermented red onions will have a sour albeit less pungent flavor than unfermented ones.