Note: I use whole wheat flour because it creates a denser coating that enables less oozing and a crunchier, sturdier crust. However, you can substitute all-purpose flour if you prefer.

In a large, shallow container, combine onion slices and buttermilk, ensuring to coat all the onions. Cover and refrigerate for at least one hour, or overnight (longer soaking will yield more tender onions).

Preheat the oven to 425°F. Line a baking sheet with foil. Remove onions from the buttermilk and arrange on the prepared baking sheet. Arrange onion slices in pairs, with each pair having one ring about 1/2 inch smaller in diameter than its partner (the goal is to nest one inside the other with just a small gap between them).

Pour 3/4 cup flour into a small bowl. In a separate bowl, beat the eggs. In a third bowl, combine the remaining 1/4 cup flour, corn meal, panko, crushed Corn Chex, salt, and pepper. Gently dip the stuffed onion rings, one after the other, into the flour, and then into the egg, and then back into the flour. Dip the rings yet again in the egg and then into the panko coating. Transfer coated onion rings to the baking sheet. Spray onion rings liberally with nonstick cooking spray.

Bake until crisp and golden, about 22 minutes, flipping once midway through. Serve with marinara or ranch for dipping.