What are the conditions for reheating dry food on Shabbat? For example, if a person placed foods such as chicken and rice in the refrigerator after the Shabbat meal on Friday night, may he reheat it on Shabbat morning?
According to all opinions, it is forbidden to place cold food directly over a flame or in an oven on Shabbat. Even though the stove or oven was turned on before Shabbat, and the food had been fully cooked, one may not place the food over an open flame or inside an oven on Shabbat. This is forbidden according to all views.
However, according to Hacham Ovadia Yosef, as he writes in his work Yehave Da’at, one may reheat fully-cooked, dry food over a “Blech,” that is, a metal sheet covering the flame. Since people do not normally cook in this fashion, placing food on a metal sheet does not resemble standard cooking, and is therefore permissible on Shabbat. Placing food on a “Blech” differs in this respect from placing food over an open flame or in an oven, which are, of course, conventional methods of cooking food. Hacham Ovadia maintains that one may place the food anywhere on the “Blech,” including the area directly over the fire.
There are other authorities, such as Hacham Ben Sion Abba Shaul (Israel, 1923-1998), who disagreed, but one may certainly rely on Hacham Ovadia’s lenient ruling in this regard.
Hacham Ovadia likewise allows reheating dry, cooked food on an electric hotplate. One may place the cold food anywhere on the hotplate, even directly over the heating element.
Summary: It is permissible to reheat dry food on a “Blech” or electric hotplate on Shabbat, provided that it is not a liquid and has been fully cooked. One may not reheat food on an open flame or in an oven on Shabbat.