Mexican onion question

[hoodat]

The Mexican groceries in this area sell an onion that is sort of halfway between a scallion and an onion. The have a green top but the bottom has a bulb about the size of a large marble. I use them like scallions but the bulb at the bottom gives them more "body" in oriental soups.
My question. Are they a special variety or just regular onions pulled when still small?