That thermos smell is living in the rubber seal

Baked-on food smell in a thermos almost always comes from the gasket or lid, not the metal, so that’s where you need to focus.

First, wash the whole thing with hot soapy water, then pry out the rubber seal if you can. That little ring holds onto grease and smells like nothing else. Soak the seal and the open thermos in very hot water with a couple tablespoons of baking soda for a few hours, or overnight. Baking soda is a base, and most food smells are mildly acidic, so it actually neutralizes them instead of just masking them. Scrub with a bottle brush after, and let everything dry fully before you put the lid back on.

If the smell is stubborn, white vinegar works too — just rinse a few times, because vinegar has its own smell and you don’t want that to be the next thing your coffee tastes like. Bleach is a bad idea on stainless steel, and those oxygen cleaners are fine but no better than the baking soda.

One more thing: the lunch box itself can soak up smell too. Wipe the inside with vinegar and leave it open to air out for a day. The thermos is often innocent — the bag is the villain.

If the smell survives all of this, the gasket is probably old and porous. Just buy a replacement seal. They’re cheap, and the smell is sealed into the rubber, not your steel.