Those small hand warmers are iron fillings, salt, water and sawdust. Add oxygen and the iron fillings rust, releasing heat over 4-6 hours.
That long time frame is great for a day of skiing, but kind of long for meal prep. Perhaps more finely ground iron would react faster, but then it wouldn’t be the same off-the-shelf, mass-produced item.
Self-heating MRE’s are water + magnesium and bits of salt and iron. The spec is to heat 8 ounces of food by 100F in 20 minutes so that seems about right for our application
The quick-acting, reusable hand warmers in which a liquid solution changes to a solid are pretty heavy for the heat you get from it.