Start by peeling your yellow or Vidalia onions, then slice them into 1/2-inch thick rings.

In a mixing bowl, combine 1/2 cup of all-purpose flour with 1/2 teaspoon of fine sea salt. Toss the onion rings in this mixture until they are well-coated.

In another bowl, whisk together the remaining flour, cornstarch, baking powder, and a pinch of salt. Slowly add ice-cold club soda to the dry ingredients, blending gently to create a batter.

In a heavy pot or deep fryer, pour enough neutral oil to fill it about 2-3 inches deep, then heat it to 375°F (190°C).

Carefully dip each floured onion ring into the batter, allowing any excess to drip off. Fry them in small batches for about 2-3 minutes per side.

After frying, use a slotted spoon to transfer the crispy onion rings to a wire rack. Immediately sprinkle flaky sea salt over the rings.