Cebollas Guisadas Choose onions about the size of shallots, but not quite as small as pickling onions. A fast way to peel them and to keep a nice shape is to put them in a bowl or saucepan, cover with boiling water, drain, then rinse in cold water -the skins just fall off, Braising is an excellent way to cook small onions. In Spain, large yellow onions are very sweet and are often eaten raw after being soaked in water for a little while -a habit that is said to go back to the Moors.