Combine all the ingredients except the onions and vegetable oil in a large bowl with 300ml/1012fl oz/1% cups warm water and mix thoroughly.
Set a large pan over a medium-high heat and fill a third of the way up with oil. (You know the oil is hot enough when you can drop a small amount of batter into it and it bubbles and floats to the surface.)
Toss a small batch of the onion rings into the batter, making sure they are fully coated. Carefully transfer the battered rings to the hot oil and fry for 2 minutes, then turn and fry for a further 3 minutes or until they are an even golden brown. Remove using a slotted spoon and drain well on paper towels. Repeat in small batches. Serve immediately.
