Slice the onion as thinly as possible (a mandolin would be perfect, but I don't own one, so I used a sharp knife.).
Soak the onion in the buttermilk for 15 minutes, or while the oil is heating.
Toss the onion slices in a plastic bag with the baking mix until well coated.
Drop the coated rings into the hot oil, fry for approximately 4 minutes, turning half way through so that both sides are the same color (should be light brown, and you will likely see the onion "peeking" through the coating.
Drain on a plate lined with paper towel.
Serve hot with your favorite dip- Ranch dressing or ketchup are both good with these.