Can you plant a whole onion that has sprouted?

I've done it a number of times...
sometimes I get good results...

It will probably grow like a scallion(spring onion) for bit, then bloom instead of making a new edible bulb like the old one.

Yes you can , results will differ.
Best thing is check and see how many new onion the sprouting one is forming.
I have split them into up to three new onions.
They do best if not multiple new onions fighting each other for space.s