My Dairy microwave was used to heat milchig food (frozen dairy “vegetarian” shnitzel with milk in the ingredients) but on a fleishig but non-ben yomo ceramic/china plate.
What if anything needs to be done with the plate? Can I continue to use it?
Do I need to kasher the microwave? Can the microwave be kashered and if so how?
Assuming that the milk was not battel (meaning, that the milk is more than one sixtieth part of the shnitzel, and therefore imparts taste to the food), you will not be able to continue to use the plate for direct contact with food.
The microwave is fine, however, and no action needs to be taken.