Peel the onions and cut them into thin rings. You can use a mandoline for uniform and neat slices. 

Toss the onion rings in wheat flour and place them in a sieve to shake off excess flour. 

Heat the oil. Check if it is hot enough by dipping a matchstick (the end without sulfur) into the oil; if it sizzles and bubbles around it, the oil is ready. Put the onions into the pot, and when they turn golden brown, remove them with a slotted spoon. Place the fried onions on paper towels to absorb the excess oil and let them cool. 

You can use the same method for chopped onions and spring onions, cut into smaller pieces.