I get that it’s the onion, but I will add that if you tried to heat anything in a cremation oven it’d be gone. Those things run at 2k+ degrees. So the cup and coffee would be legit vaporized.
Nah, water wouldn't vaporize instantly, it takes a while. But the mug would definetly crack from thermal shock. Unless it would be a metal mug in which case it might work. The coffee would probably be boiling in seconds.
Camp fire mug would do it but it would boil out the contents in a moment from the near instantly achieved rapid boil. And not fully evaporated just nuked into steam as it ejects / "explodes" from the cup. Would look a lot more violent then rapid nucleation.
It's not like you throw the cup straight in though. Just open the door slightly and bring the mug close. The metal will conduct the heat well and considering that water has a pretty decent heat capacity it won't blow up instantly.