[upthetump]

just reading the Wilko's website on the red sun shallots page. it says "Try growing mint with your onions to deter the dreaded onion fly!"

anyone see any pitfalls with this? 

http://www.wilko.com/onion+potato-seeds/wilko-shallots-red-sun-500g-714/invt/0180639

[Goosegirl]

Not as far as a companion plant, but don't plant it in the border or put a pot of it on top of the soil as it will romp away with roots coming up all over. Try making an infusion and spray it over a selected patch of your onions and see if there is any difference to the un-sprayed ones.