Slice onions into 1/4-inch rings and separate.
Whisk buttermilk with the egg and a pinch of cayenne until smooth.
Whisk together flour, cornmeal, paprika, garlic powder, and salt in a separate bowl.
Dredge rings in the dry mix, tapping off any excess.
Dip rings in the wet batter, letting excess drip back into the bowl.
Coat rings in the seasoned breadcrumbs and cornmeal for a crunchy crust.
Preheat oil to 190°C (375°F) in a wide pot.
Fry rings in batches 2–3 minutes until deep golden, flipping once for even color.
Drain rings on a rack for 1–2 minutes to shed excess oil.
Season hot rings with salt and finish with a squeeze of lemon over them.
Serve immediately with lemon wedges and your favorite dip.