Heat oil in a frying pan on high heat. When the oil is hot add the sliced onions and salt.
Saute the onion on high heat until it gets a glassy / translucent look for about 3 - 6 minutes.
Reduce the heat to medium and while stirring infrequently to avoid uneven burning, reduce the water content of the onions. This is a tiring toil so be prepared to shed a tear or two until. It shouldn't take long though. When the first big batch of steam from the onion subsidies in about another 3 - 6 minutes we may hear the onion sizzling in the oil.
Reduce the heat to minimum and stirring every now and then, let it gently cook for about 15 - 20 minutes until it gets the brown caramelized color we wish to achieve. Use it immediately or can it for later use.