It's elephant garlic gone to bloom. Your description fits it to a tee. I have it in my garden as well and let it go to flower just because the bees like it so much.
Aha,
I missed the flattened leaves in the OP.
The walking onions have round leaves.
Interesting this year, in that some of them not only have the bulblets, but another shoot is starting out past them and there is a typical allium (onion) type flower. I've been growing these for darn near 10 years and never seen that before.
