Adjust an oven rack to the lower third position but do not turn the oven on.
Put the whites into the bowl of a stand mixer. If whites are refrigerator temperature, let them wait at room temperature ...
Finely chop the garlic and mash it together with the salt to make a paste. Scrape it into a medium bowl and add the onion, if using. Add the mayonnaise, buttermilk, parsley, chives and black pepper ...