Sift the flour, Cajun seasoning, and a pinch of salt into a large bowl. Make a well in the centre and add the yolks and oil. Slowly whisk in the milk to make a smooth batter. Chill in the fridge, covered, for 20 minutes.
Slice the onions into rings about 1.5cm in diameter. Separate the rings and set aside.
Heat the oil in a deep fat fryer to 180c, or you can use a large saucepan and fill it 1/3 with oil. Heat until a small piece of bread dropped in the oil turns golden brown in 60 seconds.
Whisk the egg white until forms stiff peaks and then gently fold it into the batter.
Dip the rings into the batter, a few at a time to coat them completely, then carefully fry in the oil in batches, for 3-4 minutes each, turning them over halfway through, until golden brown. It's important to only cook a few at a time, because if you overcrowd the pan, they won't cook properly.
Lift out with a slotted spoon and transfer to a plate topped with three sheets of kitchen paper - this will soak up any excess fat. Keep warm while you continue to cook the rest of the rings in the same way. Serve with tomato ketchup.