Heat the oil in a large saucepan to 170°C. To test, add a cube of bread to the oil; if it browns in 1 minute then it’s at the correct temperature for frying.
Combine the 2 flours in a large bowl and season with freshly ground black pepper. Add the onion rings and gently mix until they are all lightly coated. Add the milk to the bowl and mix until all the onion rings are slightly sticky.
Deep-fry in batches until golden brown and drain on kitchen paper, then serve.