Lost the lot

So sorry to hear this, they are utter bar stewards. 
I feel your pain having lost all my leeks twice  and garlic once. I now use a mesh barrier on leeks and garlic pretty successfully.

That sucks. Sometimes gardening can be really disheartening. Hopefully they don't come for my leeks, they're interplanted between other crops so would be difficult to net.

I've just looked at my spring sown onions and there is some of the tell-tale rows of white spots on the leaves. No obvious damage yet but there likely will be.
Leeks seem to be the main crop that would help them to overwinter. These should miss the first wave in spring and need covering from September on to guard against the second generation. I have noticed red lines in them and just cut them out. So in future, I will make sure any leek debris is clear of pupae or disposed of in LA compost bin. This hopefully should be good enough for me as no-one else nearby seems to grow too much.
On an allotment, I would make other plotholders aware of the problem and also cover onions till end May. So 1 set of covers should do for both crops.
Sets may be better to grow than seeds? Also my Autumn planted sets seem unaffected, I'm starting using them now, bit like extra large spring onions.

Sorry to hear your bad news Boss, never heard of Leaf Miners, must google them.

There is also beet leaf miner which attacks beetroot and chard, two things which I now have to grow under netting.

There is also beet leaf miner which attacks beetroot and chard, two things which I now have to grow under netting.

...and also a celery leaf miner which attacks celery and parsley. Found some on my celery today. Not as destructive as the beat leaf miner but still annoying.