Finely sliced onion
Good quality cooking oil
Slice onions thinly across the grain (as for onion rings) and discard any thick pieces.
Heat oil in wok and fry in batches until golden to medium brown. Remove onions with a holed spatula or flat sieve.
Dry on paper towels and store in an airtight container when cold.
When the oil is cold bottle as required.
The quantities are not critical. I usually have just under a ½ wok of oil to about 6 large onions, but more can be used – the oil will just get more fragrant. It will take a while to go golden but be careful as it goes from light to dark brown very fast. Alternate tossing and pressing in plenty of paper towels to remove as much oil as possible. Store when completely cold.
More step-by-step photos here.