How to get rid of onion smell from the chopping board ?

Scrubbing your wooden cutting board with salt and crazy hot water works. To prevent your board from picking up odours in the future make sure it's well seasoned - Here's how: http://karyosmond.com/maintain-wooden-cutting-boards/

Lemon and salt work for wooden boards. Any others, I run through the dishwasher.

My favorite new cleaner for stuff like this (that comes into contact with food) is organic cleaning products from Freshana Organic Solutions. I swear by the stuff now. Will totally get rid of onion smell.

I agree with Madame Sel, course salt & half a lemon works best. Sorry Mensaque, but I must disagree with you. Dish detergent can get into the board leaving a residue behind.
The best way to keep a board from developing an odor is to keep it properly oiled. Use food grade mineral oil, beeswax or combination of both. Food grade mineral is cheap & easy to find. All you have to do is rub it on & let it soak in. Do all sides of the board. I would recommend that you do it every time you use the board until it develops a good finish. When finished chopping, rinse the board with warm water, use a stiff brush to remove anything stuck on. No soap. Soap will break down the protective oil. Dry the board then apply the mineral oil & leave it to soak into the wood.
Make sure to use Food Grade Mineral Oil as other oils (olive, nut or canola) can go rancid on the board.

Wash it with dish detergent and then scrub some used ground coffee from the filter and rinse.Works well on plastic too,and even on your hands.

I keep a shaker jar of baking soda with my cleaning supplies. I use it along with my dish detergent to clean my plastic cutting boards always. It's a gentle abrasive that removes any residue and it deodorizes, too. In fact, when I wash my hands after handling garlic, I sprinkle a little on with my hand soap.

Thank you will try it.

Have you tried scrubbing it down with kosher salt and a half a lemon? That works for me on wood boards. Or try some diluted white vinegar. Spray it on and leave for a few.