Preheat the oven to 450 degrees . Line two large baking sheets with parchment. Transfer the larger onion rings to a bowl of ice water; reserve the smaller rings for another use. In a wide, shallow dish, add 1 1/2 cups flour. In a second wide, shallow dish, whisk the buttermilk, remaining 1/4 cup flour, 1 tbsp. coarse salt and the curry powder, if using, until well combined. In a third wide, shallow dish, mix the cornflakes and breadcrumbs.
Working with 1 ring at a time, remove from the water adn coat in the flour, then soak in the buttermilk mixture, then coat in the breadcrumb mixture, shaking off any excess. Transfer to the prepared baking sheets in a single layer. Spray generously with cooking spray.
Bake the rings for 25 to 30 minutes, turning once with a fork and rotating the pans (top to bottom) on the shelves. Serve hot.