Somebody else had the same problem on answerbank a few weeks ago. If I remember rightly they were told to cut an onion in half and leave overnight. I tried this after cooking fish in my kitchen and yes it worked, but no it didn't smell of onion either.
I gave this advice in answer to a similar query last November, and the questioner posted that it worked a treat.
Clean and kill off the bacteria using bicarbonate of soda.
Thwn get rid of the smell by leaving a couple of bowls of malt vinegar standing in the boot overnight. Just remember not to drive off with them still there in the morning.