The Hilchos Shabbos shiur at the kollel is learning th sugyos of Bishul, We learnt about the Halacha of Bishul Bechama cooking with the sun. The Gemara teaches that it is perfectly permissible to cook with the sun, because it is not the derech of Bishul. There is a fascinating machlokes between Reb Moshe and Reb Shlomo Zalman if nowadays that people use microwaves is that considered similar to Bishul of Aish which would be assur min hatora or is it not the normal way to cook and therefore although it would be assur miderabanan on a Torah level it would still be Mutar. The reason being that because it is not the derech of cooking eventhough one could technically cook it is still not the normal way. Reb Shlomo Zalman holds that it is only assur derbanan, where as Reb Moshe says it is Assur min Hatorah. This Machlokes has other ramifications as well in terms of meals that come with the package that can self heat, according to Reb Moshe one could argue that this would be assur min Hatora to cook with the chemical reaction heat, however others maintain that even according to Reb Moshe it is not the derech to cook with the self heat pack and Reb Moshe was only discusssing a microwave which became the derech.
