Last year I had a load of spare onions and little time so I experimented; successfully. That time of year is approaching so I thought I would share.
Prepare onions by top and tailing, removing their jackets and washing.
Put onions through coarse mincer (you know the old Spong hand meat mincer) chopping as necessary to get them down the mincer throat.
Fill jars loosely with minced onions.
Top up with ordinary vinegar, doesn't take much.
After a few weeks you have a sort of onion relish which has kept well for a year so far. It is a little bland but nice with fish and chips and salads, the sort of things you would have a picked onion with I suppose. If you wish you could tart it up with spices, cider vinegar, etc. but it really is quite acceptable as it is.
Quick and simple, it take longer to wash up than to process and prepare a few jars.