No an onion, or at least the bit of it you cut up to eat, is a root. It grows under the soild and will also contain the "seed" of the plant.
A onion is a modified stem
Onion is a modified stem because it is a stem which grows underground
Onions are alliums and they are bulbs that grow underground.
Onion roots emerge from the base of the condensed stem.
An onion, strictly speaking, is a 'bulb' and is a stem.
white
stem
Onion is an underground stem called a bulb. It grows horizontally below the surface of the soil, from which roots grow downward and shoots emerge above ground.
Carrot, Sugarcane, Turnip & Onion all these are underground stem.
a bulb
its a bulb
aerial stem