Many plotholders have had problems with their onions this year. The leaves have become distorted and do not grow upright but may lie along the ground instead. There may also be light green or grey marks along the foliage. This damage is caused by the larvae of the leaf miner which is a fly that lays its eggs near the base of the onion bulb.
Once a plant is infected by leaf miner there is nothing that can be done and the plant should be destroyed. The best method of preventing leaf miner is to prevent the fly from laying its eggs on or near the onions in the first place. This could be done by using an insect-proof fleece/mesh to cover the onion sets or by planting the sets after the danger period for leaf miner has passed .
Read the following article for more details on the allium leaf miner and how to prevent it: Allium Leaf Miner