The Onion

I used to have a furnace at 815 degrees C for lab testing. Tried a toasted ham and cheese sandwich in it once, got charred bread and unmelted cheese in the middle.

Basically the heat flux is so much that you can't transfer it through the food fast enough to cook the inside before the outside burns. Which is why you can't turn up your oven to twice the temperature and cook your food in half the time.