Separate the slices of onion into rings. Set aside for a moment.
For the Buttermilk Mixture - In a large bowl with a flat bottom or casserole dish, add buttermilk. Add the onion rings to the buttermilk.
To the bowl, add hot sauce (or tabasco sauce), salt, and pepper. Mix the onions into the buttermilk so that the onion rings are evenly coated with the mixture. Set aside.
For the Flour Mixture - To a medium size bowl, add pastry flour and cayenne pepper. Stir together. Set aside.
For the Panko Bread Crumbs - To a medium size bowl, add panko bread crumbs. Set aside.
Coating the Onion Rings - Mix up the onion rings again in the buttermilk mixture. Add the onion rings, one at a time to the flour bowl and cover the onion completely with the flour.
Then dip the floured onion rings (one at a time) back into the buttermilk mixture.
Next, dip each coated onion ring into the panko bread crumbs covering each ring completely with the crumbs. Set the breaded onion rings aside on a plate.
Frying the Onion Rings - Heat a large skillet on medium heat and add canola oil.
Depending on the diameter of the skillet, more oil may be needed. Try to use enough so that the oil is about an inch high from the bottom of the skillet.
To the pan, add a pinch of flour. When it starts to bubble, it's time to add the breaded onion rings.
To the skillet, using a cooking fork, carefully add the breaded onion rings (about 4 or 5 at a time). Cook in batches so not to overcrowd the pan.
When the bottom side of the onion rings turn a golden color, using a cooking fork, carefully turn the onion ring over to the other side.
At any time, if the oil starts to get too hot (smoking; or sizzling too much), turn the heat off and move the pan onto an unused burner.
When both sides of the onion rings turn a golden color, remove them from the pan onto a paper towel-lined plate to drain off any excess oil.
Continue the cooking process for the remaining breaded onion rings. Salt the onion rings to taste.
Serve warm (let the onion rings cool a little before eating them because the onion on the inside gets very hot).
Serve with ketchup or your favorite dipping sauce.