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Tip the vinegar, sugar, 50ml water and 1 tsp salt into a pan, bring to the boil, then remove from the heat. Add the onions to the pan, cover and leave to cool and pickle. Will keep for 24 hrs.
Pour the oil into a large pan until it's no more than a third full and heat to 180C. Mix the flour, cayenne pepper, ground coriander and 1 tsp salt together in a bowl and set aside.
To make the batter, combine the flours and bicarbonate of soda in a bowl, then slowly whisk in the lager. Drain the pickled onion rings and coat in the spiced flour mixture. Carefully dip them, one at a time, into the batter, then fry in the hot oil for 2-3 mins until golden and crisp. Drain on kitchen paper and season with the paprika and some sea salt.