Looks like a solitary Agapanthus .

going by the lack of leaves I'd say it is one of the ornamental alliums. I cant bring the name out of the depths of my brain.

does it smell oniony?

possibly allium schuberti arctic snow

Agree, some form of Allium. Maybe a bit tall for A. schubertii though.

Could it be Allium scorodoprasum 'Art'?

I was also thinking of a similar one I had a few years ago, Allium 'Hair' which had lots of little bulbils round the crown rather than seeds.