Shamelessly inspired by "microwave glow block" (link).
I don't know if there are any phosphorescent microwave-absorbing dyes that would serve our purposes.
This dish is, instead, made of really thick shatterproof glass impregnated with vacuum-filled lightbulb-like cavities. Each cavity has a
small piece of tungsten suspended as it would be in a lightbulb.
When the microwave is turned on, the food cooks, and is illuminated by the random microcavities that absorb the microwaves, glow white-hot, and radiate the energy back as light.
Caution: dish will be hot when you take it out--please use the heat-proof non-glowing handle.