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cromniomancy

n. divination by onions or onion sprouts.

Wikipedia
Cromniomancy

Cromniomancy is divination by onions. It is usually done by interpreting their sprouting behaviour, after some kind of ritual to state the topic of the divination. This often involves inscribing the onions, dedicating them on an altar or something similar. But cromniomancy is/was also done in idiosyncratic way not involving sprouting them.

It is not really known why the onion was chosen for that kind of divination, not other fast-sprouting plants. A look at other divination methods shows that plants used for divination have or had some significance as important food source and/or medicinal herb. The onion is rich in Vitamin C and was important because of this, and it was widely used as medicinal herb against colds and other infections.