This is Allium x cornutum, a topsetting onion hybrid. One of the parents is Allium cepa, but the second is as far as I know unknown! I dug it up today, replanted many of the smaller bulbs to see how much they produce in one year and also some of the larger ones. The rest will be eaten!
This one originating in Croatia  was the only one of 3 accessions from Gatersleben in Germany that has proven very hardy here (although a French accession survived a few years; the other was from India)! The picture below shows all 3 accessions received in August 2009:

Croatia only flowered once (picture) in 2012 and  it had only one flower head. It has small bulbils and pinkish flowers.
These onions below were growing very densely in a patch about 25 x 15 cm..

In summer 2017, I found  an onion called  «Sint-Jansui» (Allium fistulosum var bulbifera  – this is the old name for Allium x cornutum, it’s not Allium fistulosum) in the Utrecht botanical garden:

Botanist Gerard van Buiten at Utrecht told me the following:

“Ah, I see you have found our “St Jansuien”! Yes, it is an old local variety, grown around Utrecht. One of our gardeners used to grow it on his nursery a long time ago. Every year on “St. Jansdag”, a box of onions was delivered at Paleis Soestdijk, where Queen Juliana and Prince Bernhard used to live. It is grown nowadays in some urban garden projects in the city”