Something to remember about onions is they are not roots.
You can't see it in the picture in the first post but the roots are under the bulb, the stringy things, maybe they were cut off.
So, what is the bulb? You see leaves sticking out - it's the base of the leaves. Treat onions like grasses, maybe like a corn crop. They need lots of nitrogen for their green leaves.
Like a lawn grass, an onion seedling can be crowded ... by every weed around! The lawn grass ... ummm, onion ... will be lost in the weeds! Some folks think of the onion as the ultimate garden companion plant. That's because it doesn't inconvenience anything!
Steve