Yesterday, at a casual asado with my in-laws in Buenos Aires, I had onions cooked for me in ash and the radiant heat from nearby coals. I was impressed by the smooth flavor. Cooking in ash is something I had done with potatoes before, but had not yet tried with onions. The reason I was given for cooking this way was the lack of space on the parilla (as it was already covered with meat, poor thing.) But the softness and evenness of the texture and flavor make it a trick worth trying, regardless.