What Is This Plant?

[Questman]

I was in my daughter's garden the other day when I spotted an amazing tall plant in the border. It was about seven feet tall, thin stalk and with what looked like an onion on the top. She said that it just popped up on its own.
Does any member recognise it from my description?
I took a photo but don't know how to show it on The AnswerBank.

[alavahalf]

Was it about 3 weeks ago we had a frost on a Thursday morning .. that was the trigger to send all my onions to seed, all 300 of them !
They looked exactly like the image of the allium last week .. although only 3feet tall.

[Chipchopper]

I remember when I was a small boy, the old lady next door planted a row of onions and for whatever reason she allowed the onions to become neglected and go to seed.
The result was just like the one in your photo but a whole row of them, one of the most bizarre things my young eyes had ever seen!