Dip a few onion rings at a time in the batter, letting some of the excess drip off. Fry a few rings at a time in the hot oil for 3 to 4 minutes or until nicely browned, turning them over halfway through cooking. Watch out for splattering. Drain on the wire rack. Season with salt. Keep the onions rings warm in an oven preheated to 200°F (95°C) while you fry the remaining onions.