Place onions in a shallow dish. Pour the buttermilk over the top and let them soak for about 1 to 2 hours refrigerated.
Preheat oven to 450 degrees. Line baking dish with foil or parchment paper. Add panko crumbs, bread crumbs and corn flakes and place half the crumbs in a large dish, season with salt. Reserve the rest for when the first batch is used up. This should help avoid clumping and help to stick to the onions.
Dip each soaked onion ring into the crumb mixture; coat well. Place rings onto two cookie sheets. Lightly spray with oil. Bake for 12 minutes or until golden brown. Serve immediately.