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-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.
How to get the best tasteSoaking the onion slices in buttermilk takes the raw edge off them so they taste more mellow.