These onions go by various names - walking, tree, Egyptian, topset and multiplying onions.
They are similar to a shallot in size and taste.
The 2019/2020 growing season was a difficult one - black aphids were just one problem to combat organically. In 2020 these are sold in sets of ten small bulbs which are ready for planting (see the last picture for a size guide). These will still grow to the usual full sized onions however. (In previous years these have been sold in sets of six medium sized bulbs for the same price).
Individual bulbs are planted and will multiply to form a clump of 6-10 or more bulbs. These can be plucked off the outside of the clump and eaten when required. Leaves and the topset/bulbets that form can also be eaten.
These onions are perennial. Once planted they will continuouly regrow by planting the bulbs that have multiplied or the bulbets that form.
Have a look at our blog for more detail on growing and eating them.