Peel and slice 3 large onions into thick rings. Separate the layers carefully.
Place onion rings in a bowl of 1 ½ cups buttermilk for 10–15 minutes. Drain excess liquid.
In a separate bowl, mix 1 ½ cups all-purpose flour , ½ cup cornstarch ,1 ½ tsp baking powder , 1 tsp salt and ½ tsp black pepper In another bowl, beat the 2 large eggs .
Dip each onion ring first in the dry mixture, then into the eggs, and finally coat with 2 cups panko breadcrumbs . Press breadcrumbs lightly to adhere.
Set to 375°F (190°C) for 3–5 minutes. Lightly spray basket with oil.
Place onion rings in a single layer, spray lightly with oil. Cook 8–10 minutes, flipping halfway through, until golden brown and crispy. Cook in batches if needed.
Serve immediately with your favorite dipping sauce, like ranch or spicy mayo.