From the experts:
Leonie is a chic French and German form of a name that exists in a range of variations from Leona to Leonia to Leon to Leo to Lionel, all newly fashionable after a couple of generations in style limbo.
Pronunciation can be confusing: The first syllable can be pronounced as lee or lay, the second as on or own, and the accent can be on either the first or the second syllable or be evenly distributed over all three syllables of the name, so that it sounds like the male name Leon with an ee at the end. Take your pick, but know that others might often pick differently.
Aunt Leonie is a major character in Proust's Remembrance of Things Past. Leonie Hemsworth is the mother of Liam and Chris and Leonie is the name picked by Monica Bellucci and Vincent Cassel for their daughter.