Peel, then slice the onions into one centimetre rings. Separate all the rings. Combine the milk with one-and-a-half cups of water. Soak the rings in this mixture overnight.
Empty the beer into a large mixing bowl. Gradually add the flour to give the consistency of pouring cream. Use chopsticks to mix – it shouldn’t be completely smooth. Allow this to stand in the fridge for 10 minutes then add some ice cubes to get it really cold.
Using a large, deep saucepan place oil about a third of the way up. Keep over a moderate flame until it reaches 180C. Test the oil by dropping a little of the batter into the pan; it will sizzle when the oil is hot enough to start frying.
Drain the onion rings well. Dust the rings in seasoned flour and shake off any excess. Lightly dip the rings in the beer batter and, with care, immediately place into the hot oil. Cook, turned as needed, until golden brown, about two to three minutes. Carefully remove and drain on absorbent paper. Cook in batches, ensuring the oil is reheated in between and strain out any floating bits.
Sprinkle with sea salt and serve.