it needs phaty acid to disolve. don't matter microwave or oven.
try puttin some weed in water in the microwave and drink it.
(nothing will happen)
then try boilin it in milk in the microwave
(you will be stonned)
thats why people use butter, milk, and whatever else that has fat in it.
and penut butter or cheese in the microwave for a couple min will not do it.
you have to give it some time for the chemesrty to work.
I never used a microwave so all what i say may be wrong, yet i don't see how it wouldn't work.

your better off using some butter in a pan and use it to make a grilled cheese sandwitch.
have fun!