You would get “Potato Onions,” which is what I grew for the first time this year. Apparently they’re also called “Multiplier Onions,” but that name doesn’t evoke any of the wonderful Frankensteinian images of a half-spud-half-onion hybrid.
For what it’s worth they were pretty decent, though they may not overtake my beloved shallots in the ‘What to Grow’ league table.
They take up the same amount of space as the shallots, and work in the same way – that is, for every set you plant, you get several small onions when you harvest.
The taste, though, is not different to your average white onion, so there’s not much to set them apart from the cheaper and easier to buy ‘normal’ onion sets and seeds.