On Shabbat it is forbidden to return any type of food to an open fire. The Rabbis were concerned that this would bring someone to stoke the fire or create the appearance that one is actually cooking on Shabbat. The question is, under what circumstances may one ask a non-Jew to return food to the fire on Shabbat. There are three cases we must discuss.
The first case is having a non-Jew return a cooked dry food to an open flame. The Be’ur Halacha (Rav Yisrael Meir Kagan of Radin, 1839-1933) rules in siman 253 that it permissible because it constitutes a Shvut D’Shvut, a Rabbinic prohibition on a Rabbinic prohibition. That is, telling the non-Jew to violate Shabbat is itself only a Rabbinic prohibition; and returning cold cooked dry food to an open flame is only a Rabbinic prohibition. When the prohibition is a “Double D’Rabanan,” we can be lenient in a case where the action is necessary for enhancing Shabbat. This would be a solution in the event that someone forgot to set up his Blech or hotplate. It is only permitted to have the non-Jew return the food to a fire that was already lit before Shabbat.
The second case is having a non-Jew return cold liquids to the fire. Here, the Be’ur Halacha quotes the Birkei Yosef (The H”ida, Rav Haim Yosef David Azulai, 1724-1806) who says, fundamentally, this is also permissible. Even though we hold that reheating cold soup constitutes a “new” cooking, however, since there is a disagreement between the poskim on this matter, we can permit it to be done by a non-Jew.
Hacham David Yosef in Halacha Berura, as well as the B’eur Halacha, add a caveat to this leniency: The non-Jew may only return the cold soup to a fire that is garuf or katum, i.e. a blech or Shabbat hotplate.
The third case involves returning a Ma’achal Ben Drosai, a food that was only half-cooked. Perhaps today we would refer to it as extra-rare, although edible. There is a disagreement between the poskim whether continuing to heat such a food, bringing it from rare to medium to well done, constitutes the Torah prohibition of cooking. We hold that it is forbidden, and therefore, a Jew certainly cannot put such a dish on the blech. However, Hacham David writes in Halacha Berura (siman 318) that it would be permissible for a non-Jew since there is a dissenting opinion.
1. It is permissible to tell a non-Jew to reheat fully cooked, cold, dry food on an open flame (ignited before Shabbat).
2. It is permissible to tell a non-Jew to reheat fully cooked cold liquids only on a blech or Shabbat hotplate.
3. It is permissible to tell a non-Jew to put a partially cooked food only on a blech or Shabbat hotplate.