https://www.cnet.com/news/gosun-shows-off-new-solar-oven-at-ces-2019/

The tech behind solar cooking works with vacuum tube technology without the use of propane or charcoal. Sunlight is focused onto the vacuum tube by parabolic reflectors. That converts nearly 80 percent of the sun's rays into heat that can build up to 550 degrees, while keeping the exterior of the cooker cool to the touch. 

That vacuum tube also works as an insulator to keep the food hot. The Fusion also uses a thermal heating element in the base of the cooking tray. The heating element needs a 12V battery to cook a full meal and is encased in a stainless steel cover.