Preventing Onion Fly

Last year my onions were infested (Sets and seed sewn) with maggots from onion fly. They even got into ones in small pots. Never had it before but as they may still be in the soil I would prefer to take steps this time to prevent another disaster.

If I plant the sets with some form of cheap weed protective black fiber with the onions poking through, would that be enough to stop it ?

Not cheap though. 

it is a step in the right direction but you also need to crop rotate!

Following on from this as a newbie, should precautions be taken to cover my onion bed with protection , Its my first year and dont actually know what the ground is like.  Other plot holders dont seem to have any protection on. 

If they're already in the soil how will the black plastic help?   

Following on from this as a newbie, should precautions be taken to cover my onion bed with protection , Its my first year and dont actually know what the ground is like.  Other plot holders dont seem to have any protection on. 

I do not use any covering but we do not have any problem with fly on our plots, but there is no telling if we will get them this year.

However we are plagued with carrot root fly instead so you cannot win.