Those look like chives rather than onions. Just start harvesting them and they'll regrow. Cut stems to the base to harvest.

Are you giving them enough water or too much? They look like onions to me, but maybe not?

The soil looks like commercial potting soil? The perlite is visible...so it shouldn't be the potting soil. If it is commercial potting soil...did it already have fertilizer in it. Did you add more?

Over-watering is often the cause for me. I used miracle pro mix and know when to water by lifting the post and feeling it get lighter. the color of the soil changes to lighter brown too. Could you be over watering. The roots must have oxygen too.

Doubt it is under fertilized. The soil looks commercial and probably has enough for that. So watering? Not enough or too much..maybe?

I grow onions in containers and follow a couple of rules. Leave the soil loose. Put soil in pot and water. Then gently push onion sets into a spot. I read the recommended in 2 inches, but I harvest early and have them an inch and a half or so. Cover loosely with soil and gently water them in.

I can easily see when the top of the soil lightens in color and lifting the pot feels lighter...that is when I water. I read here that the main problem people have is too much water...or too little fertilizer. It looks water related to this non-expert....so caveat on that.