Test the oil with a small pinch of panko; if it bubbles and fries, then it is hot enough. If it turns black right a way, it is too hot. Using tongs, transfer a handful of onions from the buttermilk to the flour/panko mixture. Toss to coat, pressing to adhere, shake off any excess flour, then carefully transfer to the hot oil. (The coating will be thin but will crisp up.)