Heat 2 inches of cooking oil in a large pan or skillet to 350 degrees.
Place sliced onions in a disposable gallon plastic bag. Add in Cajun seasoning. Close bag and shake to distribute the seasonings. Add one tablespoon buttermilk and mix it around in the bag. This adds something to make the flour cling better.
Add in enough flour to coat the onions. Close bag and shake well.
Drop in the heated oil in batches. Cook onions until nicely browned and crispy. Remove to paper towel to drain.
Repeat until all onions are cooked.