For standard, straight-forward declarations, I would say "There are no onions," not "There's not an onion."
However, for empahsis, you can say "There's not a [whatever] in the [whatever]."
There's not a soul in the room. (No one is there.)
There's not a parking spot to be found. (There are no parking places anywhere.)
Who ate all the candy? There's not a single piece in this bag! (There are none.)
Does that make sense?