Compare to Ornithogallum umbellatum.

Ornithogallum umbellatum (Star of Bethlehem?) seems to grow many flowers to a stalk and the look down. The one I have is one is one per stalk and probably one per bulb. The flower is face up.

A closer photo of flower might help someone else ID your plant.

too vigorously. remove!!!

The picture shows Ornithogallum umbellatum - no doubt at all. In the photo you can see the multiple flower heads per stem (even though only one is open) and they face up.

I agree with floral (who by the way is quite good at this plant ID stuff); it's Ornithogallum umbellatum.

I am seeing this now. I thought Star of Bethelem was a rhizome not unlike Lily of the valley while these are bulbs. Ok, so Ornithogallum umbellatum it is. One thing though, after 40 years, It certainly is not invasive here in my garden. They do wander a bit and for me, it is not unwelcome,

I can now enter the name: Ornithogallum umbellatumrather than "white onion like thingie" Now that I hear that it is invasive, I guess it is going to live up to it...

It's not so invasive in a lazy gardener's yard. It's when you're actively relocating other garden plants is when it spreads the most. 'Hitchhikers' can be common.
mike