When choosing a thermos, check the label to be sure it's vacuum-sealed and rated to retain heat for at least 6 hours. This will ensure that your food will retain enough heat to keep your food at a safe temperature. Stanley and Thermos brand thermoses have versions that fit those specifications.
To slow cook food in a thermos, boil the food or liquid in a pan for 2-10 minutes, depending on the recipe. Once it's boiled, pour it into the thermos, replace the lids and enjoy your day until it's time to eat.