Peel and slice the onions into thin rings. The onion slices should be thin enough see thru.
In a shallow pan, pour buttermilk over the onions and let soak for at least an hour. The onions do not need be completely covered by the buttermilk, but should be touching it.
When ready to fry, heat a quart of oil in a deep fryer equipped with a temperature control, or in a pot on the stove to 350°F.
In a large flat dish, mix 2 cups of flour, salt, black pepper and ground red pepper.
Working in batches, grab a tong full of onions, tap off the excess buttermilk and drop into the dredging mixture to coat. Tap off the excess flour and drop the coated onions into the hot oil.
Onions are cooked when lightly golden brown.
Remove from the oil and place on a oven safe rack set over a sheet pan lined with paper towels to drain. Lightly salt to taste. Transfer them to another paper towel lined sheet pan and keep them warm in a 200 degree oven.