Use nemasys on the patch you grew your onions in last year. That should hopefully destroy any maggots that are in that soil stopping them maturing.

http://www.nemasysinfo.com/gyo.shtml

Plant this years onions in a different area, use nemasys on that, and then use enviromesh to form a physical barrier around and over the onion patch preventing surviving onion fly reaching the onions.

Then pray.

Crop rotation is important. All crops should be moved around one third of the plot each year. I wouldnt use chemicals, as rotation should solve the problem.

Only trouble with rotation is that flies that hatch from the old patch go looking for fresh onions in a new patch.

THANKS GUYS