Potato Onion (Allium cepa var. aggregatum)
Potato onions are related to shallots, but are mostly round like ordinary brown onions. They are not sown from seeds, but from onion sets, which will split into several new onions when the plants grow.
Potato onions often keep very well and also flower seldom. Onion sets for the next season can easily be stored the whole winter.
When you plant large sets they will mostly split into several small onions that you can keep for sets for next year. If you plant small sets they will instead give a few large onions that can be used for cooking. However, this varies with environmental factors such as water and nutrition.