Cromniomancy

1660, Robert Burton, The Anatomy of Melancholy in Brand Popular Antiquities (1844) iii. Burton […] speaks of "cromnysmantia," a kind of divination with onions laid on the altar at Christmas Eve, practised by girls, to know when they shall be married, and how many husbands they shall have. This appears also to have been a German custom.

cromniomancy consisted of laying onions on which certain names had been written upon the altar on Christmas Day; when these were planted the one which sprouted first would give the required indication.