Why does a wooden cutting board still smell like onions three hours after you washed it?

Wood is porous. Onion compounds soak into the surface and soap alone cannot pull them out.

The scrub

Coarse salt and half a lemon. Pour a tablespoon of kosher salt on the board, cut a lemon in half, and scrub the board with the cut face for about a minute.

The salt is the abrasive. The acid in the lemon breaks down the sulfur compounds. Together they pull the smell out of the grain instead of just off the top.

Rinse with warm water and stand it on edge to dry. Do not lay it flat on the counter while wet.

Garlic and fish, same drill

It works on both. For fish, add a pinch of baking soda to the salt. I learned that one from a neighbor who ran a seafood counter for 20 years, and I have never found a better fix.

Skip bleach. It fades the board, dries the wood until it cracks, and leaves its own smell behind.

Plastic boards

Plastic holds odors longer than wood, which surprises people. The scratches trap the compounds and the surface is too slick for salt to bite.

For plastic, make a paste of baking soda and water, spread it on, and let it sit for 15 minutes. Baking soda neutralizes the odor rather than masking it.

The rotation habit

I keep two boards now, one for onions, garlic, and fish, one for everything else. That single change cut the problem in half without any scrubbing at all.

If you only own one board, cut onions last and scrub right away. Onion compounds set in faster the longer they sit.