Toss the thinly sliced onion with the flour in a bowl until all coated.
Heat the vegetable oil in a small pot or frying pan over a medium heat. Once hot, add the sliced onion - you will probably have to work in batches so you don't overfill the pot.
Leave to fry, stirring occasionally - the oil will bubble up, the bubbles will recede and the onions will start to turn brown. Once they are all mostly brown, remove from the oil with kitchen tongs to a plate lined with paper towel to drain.
Repeat to fry all the onions.
Store excess, cooled, fried onions in a lidded container in the freezer.