https://en.wikipedia.org/wiki/Haybox
A hay box, straw box, fireless cooker, insulation cooker, wonder oven or retained-heat cooker is a cooker that utilizes the heat of the food being cooked to complete the cooking process. Food items to be cooked are heated to boiling point, and then insulated. Over a period of time, the food items cook by the heat captured in the insulated container. Generally, it takes three times the normal cooking time to cook food in a hay box