You Don't Need a Thermos. You Need a Heat Pack.

You don’t need a thermos, but you do need something that traps heat — an insulated lunch bag plus a heat pack works.

The trick is to keep the hot and cold stuff from fighting each other. Put your warm main in a small metal or glass container, and one of those microwaveable heat packs (the kind for sore shoulders) next to it inside the insulated bag. Preheat the container with boiling water for a couple minutes first, dump it out, then fill it with your hot food. That holds heat way longer than you’d think.

For the cold side, keep it in a separate small container with a little ice pack, and place it on the other side of the bag. If your lunch bag has no divider, just put the ice pack between the hot and cold zones.

The honest caveat: this works for a few hours, not all day. If you’re packing at 7am and eating at 1pm, you’ll be eating lukewarm food. If that’s a dealbreaker, buy a heated lunch box. It’s basically a mini slow cooker and it solves the whole problem with zero cleverness.

Either way, skip the thermos. Just get the temperature zones working for you.