Cromniomancy is a form of divination by means of onions.
It was usually practiced on Christmas Eve to obtain information about absent persons.
One method was to inscribe the names of absent friends on individual onions and leave them undisturbed on a table until they began to sprout.
The onion that sprouted most rapidly indicated that the person whose name had been inscribed on it was enjoying vigorous health.
Another method was to obtain an answer to a question by inscribing alternative answers on individual onions; the correct answer was said to be that on the onion which sprouted first.
Some also believed that wishes would come true if they burned onion skins on a fire.