Great use for a thermos!
The principle is an old one, popular in days gone by particularly in Eastern Europe.
There are books written on the subject... you might find recipes you'd like to try.
The old way was to use a straw-lined box and to put a heated pot with food in the box, to be eaten later in the day... then on goes the insulated box lid.
The idea was updated by lining the box with a loose sack containing styrofoam beads.
I've seen plans where people use their favorite pot (wrapped in plastic) as a form and filling the space around it with one-part foam, etc.
But just for breakfast, you really hit the sweet spot!
-AC