Slice 4 large onions into 1/4-inch rings. Place in large bowl and cover with buttermilk. Season with salt and pepper. Cover with plastic wrap and refrigerate for 30 minutes.
Place 2 cups flour in a shallow dish. Season with salt and pepper. Dredge onion rings in seasoned flour. Dip again in buttermilk, then in seasoned four.
Fry in batches, in 350 degree oil for 2 to 3 minutes until golden.
Drain on paper towels. Let oil come back to temperature before frying next batch.