Firstly, you could protect this year’s sowing with a covering of Environmesh, which will exclude the egg laying adult onion flies, or you could grow autumn-planted onion sets which normally escape attack. ‘Red Baron’ and ‘Jetset’ should give good results. Varieties suitable for autumn planting include ‘Senshyu’ and ‘Radar’ and these will crop and be ready some weeks before the normal spring-planted or sown crop.