Line a baking sheet with paper towels; set aside. Place onion rings in a large bowl of ice water for 10 minutes; drain and pat dry. In a large bowl, whisk together flour, cornstarch, baking powder, salt, and seltzer.
In a heavy 5-quart pot or Dutch oven, heat oil over high until a deep-fry thermometer registers 375 degrees. Working in seven to eight batches, place onions in batter; using your fingers, lift out, letting excess drip off. Carefully place in oil, making sure not to overcrowd pot; cook until golden brown, 2 to 3 minutes.
Transfer rings to prepared baking sheet, and season with salt. Keep warm in a 250-degree oven. Return oil to 375 degrees before starting next batch.
Once all the onion rings are done, place on a platter. Very carefully place parsley in pot (oil will spatter), and fry until crisp, 10 seconds; remove, using a spider or slotted spoon, and season with salt. Sprinkle onions with parsley.