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Put the onions in a large bowl, pour over the buttermilk and cover with cling film. Leave to soak at room temperature for 30 mins to 1 hr.
Put 100g of the plain flour on a large plate and stir in the cayenne with a little seasoning. Drain the onion rings, then coat in the seasoned flour.
Half-fill a large saucepan with the oil and heat until it reaches 180C on a probe thermometer (or until a piece of bread crisps in 20-30 secs) or heat a deep fat fryer to 180C. Make the batter by combining the remaining plain flour with the cornflour and a pinch of salt. Make a well in the centre and pour in the lager. Stir to combine until smooth.
Dip each onion ring in the batter and fry for 3-4 mins or until crisp. Drain on kitchen paper and sprinkle with the salt.