Heat oil in a deep pot or fryer to about 380-400 degrees. You can test it by dropping a small piece of onion in oil, if it bubbles and bounces the oil is ready. Or heat butter in a skillet and let it begin to turn brown and foamy
Slice onion thinly and separate into rings. Place some flour into 2 separate bowls and season well with seasoned salt. Fill 3rd bowl with buttermilk. Arrange bowls in a line with buttermilk in the middle. Dredge rings, 3-4 at a time, into flour, then buttermilk, and then flour again.
Drop battered rings into hot oil in batches to fry, or cook in butter in skillet. Cook onions until crisp and turn over to cook other side, working in batches. Cook 2-3 minutes if frying, or 8-10 minutes if sauteing, until rings are golden.
Drain on paper towels and sprinkle with kosher salt. Serve warm. Can be placed in 200 degree oven to keep warm while cooking the remainder of rings. Plan on 4-5 rings per person