Cut 1 cm slices across the onion and separate each ring.
To make the pickle, put all of the ingredients in a large pan with 375ml water, bring to the boil, then simmer until the sugar dissolves. Add the onions and cook gently in the pickling liquid for 2 minutes. Turn off and leave to cool.
To make the batter, warm the beer to blood temp then drop in the yeast and leave for 2 minutes. Transfer to a mixing bowl and, using a whisk, combine the remaining ingredients until smooth. Cover with clingfilm and leave in a warm place to ferment for approximately 3 hours, until doubled in size and bubbling. Whisk everything back together. You may need to add a litle more flour or water.
To cook the onions, heat a fryer to 180C (or heat oil in pan until a piece of bread browns in 30 seconds). Take a large handful of onions (draining the pickling liquid as you go) and drop them into the batter. One by one, shake off a little of the batter and place them in the fryer, being careful not to overload. Fry until golden brown and crisp. Drain onto kitchen paper and season with salt.