Soak it in baking soda, then let the sun do the rest

Baking soda soak kills the smell, and sunlight finishes the job.

Plastic is porous, and onion oil and fish oils get absorbed deep into the material. Soap alone won’t touch that; you’re just washing the surface. So stop scrubbing and start soaking.

Fill the lunch box with warm water and a couple tablespoons of baking soda. Let it sit overnight, or at least a few hours. Then wash with normal dish soap and rinse. If the smell is stubborn, follow up with a vinegar rinse—half water, half vinegar, swish it around, then rinse.

Then the secret weapon: direct sunlight. Put the box outside in the sun for a few hours. UV light and fresh air break down and lift out the smelly compounds that soaking can’t reach. This works absurdly well.

If it still smells after that, the plastic itself is permanently flavored. Toss it and get a glass or stainless container next time. But the soak-and-sun combo will save most lunch boxes.

Future sandwiches will thank you.