There is no way of growing alliums without covering from the moment the foliage shows if you have allium leaf miner, sorry.
Leeks need to be covered from the moment they are planted, ditto overwintering onions and onion seedlings. They will rip them to shreds unfortunately. When I got it about 10 years ago I tried everything and just gave up as they live in the soil and compost.
After years when I redid the whole allotment I bought in fresh topsoil and could net seperate beds so restarted growing them. I didn't even take the nets off to weed, I just slid my hand under to pull what I could get hold of.
One year my neighbour pulled all his, and I pulled all mine [100% clean] and I hung mine in the polytunnel, and his were all chucked straight in the compost bin as his were unsaveable.
The very next week I was sitting in the polytunnel and looked up and noticed a disturbance in my lovely clean onions and in one week, they had got to them, laid eggs, which had hatched and were eating them from the roots up. So I had to process the whole lot, I lost half the volume of the crop as I cut out all the bad bits and cooked the rest and froze it. All the bad bits went into the municipal compost bin to try and get rid as they use hot composting and hopefully that should kill them off.
So you have to be diligent I am afraid. I bought a job lot of fine mesh on amazon when I redid my allotment and I cut it to fit each of my pallet collar beds and bought blue pipe to keep it up. It is the only way to get clean onions, and you must make sure you don't grow ornamental onions where they can overwinter, and you must move the crop around the plot and leave each area after alluims without alliums for about 3-4 years to allow time for them to either die or go elsewhere. They are really destructive.