use butter and oil to hinder sticking and burning start and youll have to o it in batched basically for hours to get best effect.add some sugar to make that lovely sweet hot dog onions.they literally need to be slow cooked four hours.then when one batch isdone turn over the next and next etc etc.
Slice (do not chop or they will be really hard to serve) with a processor. Oil in bottom of huge pan with lid. Fill wiith onions. Add water to 1/4 depth and turn on low. Stir at 10 min intervals, allowing steam to escape. When all soft you can turn up the heat to drive off the residual water and keep turning over to give a fried flavour. Don't worry about browning them, you can do that when they are being heated at the event.