[mdjlucan]

Why is my onion like this

[mumofstig]

It looks as if the outside sheath has dried up, which is stopping the new leaves from growing through.

Can you gently split or cut the outside layer to release the inner ones?

[DD.]

I think it's leaf miner damage:

[snowdrops]

Gurgle onion eelworm & you get lots of photo's that look just like yours DD & md, we've had it on our site for the last 3/4 years

[Salmo]

Look at my post Curley Onions. You need to carefully cut open an affected leaf and find the grub. You may need a magnifying glass. The important question is whether it is a maggot or a caterpillar.

[RJR_38]

Looks exactly like the allium leaf miner damage we get here in the SE now 

[sunshineband]

My onions look just like that pic DD posted too, and there are small white grubs inside the leaves (I have cut one open to see)

If it is onion eelworm, would there be grubs like this?

So... Allium leaf miner.

The next step is... what?

[Blewit]

Sorry to say it definitely looks like allium leaf miner. Unfortunately the maggot seems to be followed by disease and rot so even only partly affected bulbs can take on an unpalatable gassy flavour as the bulb breaks down. I've found growing under 0.6mm enviromesh is the only effective prevention.

[sunshineband]

I noticed that last year. Only a few bulbs were affected and we burnt the worst but the ones with light damage we tried to use...not this year!!!