Holy Sprouting Onions, Batman!

I just noticed an 8" stem coming out of a neglected onion sitting atop
my fridge, which I'm inspired to plop in the garden.  If I plant this
large yellow onion, which seems eager to become a plant, what will I
get out of it?  Will it produce a cluster of onions like garlic?  Grow
a solitary onion?  I've never grown onions before, so I'm pretty
clueless here.