1 cup of all purpose flour
1 and 1/4 cups of buttermilk
Slice the onion and separate into rings.
soak the rings in ice water for two hours.
drain and dry with towel to remove excess moisture
mix flour, salt, baking soda, egg and buttermilk until well blended for the batter.
heat oil (I have no idea to what temp. I used my "super burner" eye on high and then adjusted the temperate as needed) -- side tip -- growing up when my dad would fry french fries or tater tots at home, he would drop a couple of popcorn kernels into the oil and when they popped, he declared the oil hot enough.
I mixed my batter in the same big bowl I soaked my onions in so it would be large enough to put all the rings in to sit in the batter while frying in batches.
I don't fry much. we never fry french fries or anything like that so I no idea about cooking time. but they cooked FAST. a couple of minutes on each side -- or until golden brown. As I said before, we don't fry much so I have little experience watching the "puff" of the batter while they were frying.
They were great with ketchup mixed with a shake of tabasco.