Keeping Foods Hot from Before Shabbat:
It is permissible to keep food on a blech or electric Shabbat hotplate from before Shabbat. This alleviates the concern that one may adjust the flame on Shabbat and solves many Halachic problems that would otherwise arise.
Reheating Food on Shabbat:
It is always forbidden to put any food on an open flame on Shabbat. Doing so constitutes a violation of Mechzay K’Mivashel-it appears that one is cooking.
Solid Dry Food:
One can be lenient and take cold, solid food from the refrigerator on Shabbat and put it on the blech. The reason is that the blech neutralizes the concern of adjusting the flame as well as the issue of Mechzay K’Mivashel.
The definition of dry food depends on the nature of the food and the amount of liquid present. A solid food containing a small amount of gravy can be considered dry. If there is a lot of gravy, even Hacham Ovadia prohibits reheating it.
Liquid Foods:
It is always forbidden to reheat cold liquids, even on a blech or hotplate. It is permissible to return a liquid that has not cooled down to the fire- if three conditions are met:
1. One uses a blech or Shabbat hotplate.
2. The liquid is still at least 80 degrees Celsius (176 degrees Fahrenheit) (According to the Menachem Sion and Hacham David)
3. The pot was not placed in the meantime on the floor or counter.
Warming Food in an Oven:
An oven has the same Halacha as a stovetop only if the Shabbat mode is activated and an oven blech insert or other distinguishing apparatus is used. Warming drawers have the same halachot as a blech on a stovetop.
If One Forgot to Use a Blech:
If one unintentionally left food on the open flame of a modern stovetop from before Shabbat, b’diavad, after the fact, the food is permissible to eat on Shabbat.
Using an Non-Jew to Reheat Food on Shabbat:
It is permissible to tell a non-Jew to reheat fully cooked, cold, dry food on an open flame (ignited before Shabbat). Likewise, It is permissible to tell a non-Jew to reheat fully cooked cold liquids-but only on a blech or Shabbat hotplate.
Congealed or Frozen liquids:
It is permitted (according to the Sepharadim) to put a dry solid food containing congealed fat back on the blech on Shabbat. However, it is forbidden to defrost and reheat a frozen soup on the blech.
