I know this sounds extreme but once your hair dries, the onion smell will be long gone.
Cut up onion in really small pieces, place in shampoo and let soak for 2weeks. Apply shampoo to hair as usual (do not remove the onion pieces). The onion recipe accelerates hair growth and gives beautiful shine to dull hair.
Never was your hair with hot water .it should be cold water all the way. Hot water opens the pores of your skin and scalp allowing chemicals of hair products to seep into your body. If you must, make sure it is relatively warm.
TILL I SPILL AGAIN ...XOXO