In a another bowl, beat egg and buttermilk.
Working in batches, toss the rings in the flour mixture, then dip into the buttermilk mixture. Allow most of the liquid to drip off, then toss again in the flour mixture. Shake off as much flour as possible and place the rings into the hot oil.
Fry for 2 to 3 minutes or until golden brown, moving the rings around in the oil to keep them separated.
Put the onion rings on a plate lined with paper towel and salt. Repeat.
Tip: refrigerate your onions and buttermilk when you unpack from the grocery store. The colder the better for making onion rings. Make sure to let as much liquid and then as much flour fall off as possible. Less breading means less grease absorption and a crisper finished product. When the breading starts to get wet and gummy, replace it with fresh mix. Heat the oil to at least 360 degrees, and fry the rings in batches.