u can mix rose water...it may help...

When you apply onion juice to your scalp add few drops of essential oil such as rosemary,lavender.. These oils will help to mask the smell of onion..and Apply lemon juice on wet hair after washing leave if for 2-3 mins and ten rinse with warm water...You can also use a citrus fruity shampoo or can also use a hair serum after hair wash.... hope it ll help :-)

Apply shampoo to the roots and conditioner to the tips of your hair and sit for a good 10 min.. the wash off ... The smell will go...

Next time mix some lemon juice to it. Right now try to wash it with one lemon juice mix in water. It might help.