Pour the buttermilk over the top and let them soak for about 1 - 2 hours, refrigerated.
Preheat oven to 450 degrees.
Line baking sheet with parchment paper or foil.
Combine panko, bread crumbs and corn flakes and place half of 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 they should stick better to the onions.
Dip each soaked onion ring into the crumb mixture; coat well.