Cut onions crosswise into 1 inch thick slices and separate into rings. Place rings in a large bowl of buttermilk and soak for at least 30 minutes. Reserve buttermilk. To make dry batter, mix together 1 ½ cups of flour, cornmeal, cornstarch, baking powder, and seasoning salt in a large bowl. Pour into dredging container. For the wet batter, mix remaining 1 1/2 cups flour in a bowl with 1 cup of the buttermilk that the onions were soaking in. Stir well to combine. ( If too thick, add more buttermilk. Consistency should be like pancake batter.) Pour into second dredging container. Heat oil, by pouring enough oil into a heavy skillet or pot to come to a depth of about 2 inches. Heat to about 350 on candy thermometer. (Or, alternately, make sure when you test an onion that it bubbles up and fries immediately.) Drain onions and pat dry. Working in batches, about 3–5 rings at a time, dredge onions in dry mixture and then dip in buttermilk mixture. Immediately fry onions in hot oil until crisp and golden on each side. Use tongs or a slotted spoon to remove onion rings from oil; drain well on paper towels and immediately sprinkle with salt or seasonings. Serve immediately with ranch dressing or any favorite condiment.
Buttermilk can be made with 4 tablespoons lemon juice or vinegar juice stirred into milk. Stir then let sit for 15 minutes.