Mix the plain flour and cornflour together in a bowl. Whisk in the water and lager until you have no lumps. Set to one side to rest for 15 minutes, whisking again immediately before using.
Heat the oil in a deep saucepan or deep fat fryer to 180'C.
Peel and slice the onions and then separate the rings.
Mix the salt with the cayenne pepper.
To cook the onion rings, dip them into the batter and transfer directly into the hot oil. Cook for 2 or 3 minutes until crispy and golden.
Drain the onion rings on kitchen paper, sprinkle with the spicy sea salt and serve immediately!
«The same batter can be used for coating and deep frying vegetables or seafood in a take on a traditional tempura style. Serve with a soy and chilli sauce.»