What would it take to create microwave energy without electricity?

An atomic bomb?

If you are not too hungry, and have some time, you could use the Cosmic Microwave Background Radiation.

LOLL!!

A coronal mass ejection from the sun creates microwave radiation.

But, on earth, is there any way to do it short of a nuclear weapon?

@Ltryptophan Yes. You need some aluminum foil and a magnifying lens in the sun-light. True microwave radiation can come from a powerful radio signal. Or a copper wire and a powerful magnet making electricity from scratch.

On a small scale but with longer time one can use magnifying glass and harsh sunlight (and extreme patience) to mimic the application of low watt microwave. If you want a fast result then just drop your food on an active magma, oh, you need the magma first.

Check out Trioboluminesence. Two pieces of material can be ht together to directly generate light, and X-rays. Maybe it is possible to generate microwaves.

Is dinner ready yet??!!

You’re gonna have to go low tech, and light a campfire.