This is the direct link that james waller was trying to link to (I think).
http://www.adoptarabbit.com/articles/packet/abcvegi.html
I can't find onion on any safe list. I would always suggest steering clear of anything you are not sure about. FAR better to be safe than sorry.
This is the biggest rabbit organisation in the UK and this may be useful
http://www.rabbitwelfare.co.uk/resources/content/info-sheets/safefoods.htm
This is quite a good comprehensive toxic plants list (it includes fresh foods too)
http://www.adoptarabbit.com/articles/toxic.html
On this list it says Onions are toxic and contain toxins, so should not be fed.
http://www.carrotcafe.com/f/veggies.html
Remember that young bunnies needed to be treated with extreme care in their diet. They also need changes made VERY gradually because they are still very vulnerable to gut problems. I personally would not suggest introducing any fresh foods until your bun is at least 4 months old, and other say 6 months old.
If/when you introduce, give a teeny tiny piece and then wait for 24 hours and only introduce one new food at any one time, watching behaviour, output, etc, to see if there are any changes (like diarrhoea, excess cecals, no poos, lethargic, or anything different at all). If everything is normal then you can increase the amount by a tiny bit the next day, and go through the same watching process each time you gradually increase it.
It's better to start off with more standard foods, particularly leafy greens, like romaine lettuce, dandelion leaves, or herbs such as parsley and basil. Those are mostly tolerated by most bunnies (but not all) the more exotic you get with food, the less likely they are to be able to tolerate it, so always start bland.