Slice the onions into 2 mm-thick slices using a sharp knife or a mandoline. Separate the slices so they don’t stick together. Pour the milk and salt into a saucepan, bring to the boil and blanch the onion slices in it for 3 minutes. Drain the onions and leave them to cool. Toss the onion slices in flour, place them in a sieve and shake off any excess flour. Pour the oil into a large, heavy-based pan (4 litres) and heat it to approx. 180 degrees. The oil is ready when it sizzles around a wooden toothpick. Turn the heat down slightly and fry the onions a few at a time in the pan until they are golden brown. Be careful not to add too many onions at once, as this will cause the oil to bubble over.
Remove the onions with a slotted spoon, place them on kitchen paper and leave them to cool.
The onions can be prepared 1–2 days before serving. Once they have cooled completely, pack them in an airtight container with a little absorbent paper.
