Peel onions and slice into 1/2" thick rings. Place sliced onions in a large freezer bag with 1 cup of buttermilk and let sit at least 30 minutes turning occasionally.
Preheat oven to 425°F.
Mix buttermilk, eggs and 2 tablespoons of flour until smooth. Set aside.
In a separate bowl, combine Panko Bread Crumbs, olive oil and seasonings.
Remove onions from the ziploc bag and drain well. Place onions in a bag with 1/2 cup of flour and shake to coat.
Separate the Panko mixture into 2 small bowls (this is important because the mixture gets damp and doesn't stick after a while). One by one, dip each onion into the egg mixture and then into the Panko mixture to coat.
Place on a parchment lined pan. Bake 10 minutes, flip over and bake an additional 10-15 minutes until browned and crispy.
Sprinkle with additional seasoning salt if desired and serve with ketchup!
(Nutrition information provided is an estimate and will vary based on cooking methods and brands of ingredients used.)