Begin by halving the onions, then slice them thinly for even cooking. A mandoline can be used for uniform slices, but it's optional.
Place the thinly sliced onions in a large bowl. Pour buttermilk over them, ensuring they are fully submerged. Stir gently and allow them to soak for 30 minutes.
Combine rice flour, garlic powder, salt, black pepper, and smoked paprika in a separate bowl. Whisk the mixture thoroughly for even seasoning.
Remove the onion slices from buttermilk using a slotted spoon, allowing excess buttermilk to drip off. Place them on a paper towel and gently pat dry.
In batches, coat the onions in the flour mixture. Gently toss to ensure an even coating, shaking off any excess flour before frying.
Heat oil in a frying pan over medium heat. Add the coated onions and fry them for 2-3 minutes or until golden brown.
Once fried, transfer the onions to a paper towel-lined baking sheet to drain any excess oil. Serve immediately for the best texture.