It should compost nicely.

I would peel off most of the outer rings and use if I can; then take that sprouting core and plant it. I do this all the time…. and it’s always hot here. I have several growing right now.

Oh, and make sure you keep the base roots with the core.

It's got a lot of green but no roots. Is that going to work if I plant it now? Also, should I cut back any of the green growth before planting it?

PLEASE DO NOT plant anything you buy at the grocery store that is not intended for planting. you can introduce dangerous pests and diseases into our environment and destroy farmers livelihoods. Millions of your tax dollars go to fighting these pests and increase the use of pesticides. instead buy certified seed onions and potatoes etc from reputable sellers online.

I guess I’m too frugal and hate to waste food. I use the outer rings to cook and the sprouting core to grow. I do the same with potatoes that are sprouting…. I plant them.

I don’t cut anything off the sprout; just watch them grow; and when I need an emergancy onion, I go pull one. The green part tastes good too.

I said earlier that I had used and planted what OP had. I pulled it/them today since the leaves were bent over. These each are about the size of an old silver dollar. As you can see it split into two twin onions and will be great for cooking. At this point I think they are called Fresh Onions.

And no, as long as the bottom (root zone) hasn't been damaged it will make new roots.