Is there bishul (cooking on Sabbath) after putting something in a microwave? That is, can I not put something on a hot plate that has only been microwaved?
Shalom, Thank you for your question, which is based on the law that (some opinions hold) it is forbidden to add a different type of cooking to fully cooked food on Shabbat. For example, one should not "boil" food on Shabbat that was previously baked (before Shabbat). Thus your question is a fundamental one – what type of "cooking" is microwaving considered? Is it "baking" or is it something else? This is of course a modern question, and I am unaware of the opinions of the major poskim. In my humble opinion though, one may certainly heat up fully cooked (dry and solid) microwaved food on a Shabbat hot plate on Shabbat itself. The reason for this is that even if we were to consider the microwave a separate form of cooking (something which I have many doubts about) we have found that nearly all the major latter day rabbis allow even heating up boiled food on a Shabbat hot plate. Even though this would seem to be a form of baking food that was previously boiled, they allow it. They hold this opinion because they consider heating food up as distinct from "baking" on the hot plate. Where the intent is to purely add heat to the food, and not to change its nature, nor to add a new type of taste, they say this is not a problem of baking after boiling. For this reason, many people heat up boiled rice on the Shabbat hot plate on Shabbat morning. [See Shmirat Shabbat KeHilchatah, chapter 1, footnote 181]. This logic would hold for our case also. Even if microwaving is a separate form of cooking, one may heat it up on the Shabbat hot plate as this is not considered as adding an additional form of cooking to the food, rather it is just re-heating. Blessings.