In the past, we have explained the rules regarding heating up foods on Shabbat. Let us now review these laws in a general manner.
Heating up food on Shabbat depends on two factors: Firstly, the food must be fully cooked before Shabbat, for if not, it is clearly forbidden to warm it up on an electric hotplate on Shabbat since doing so would continue its cooking process and this constitutes a Torah prohibition.
Secondly, we are only referring to warming a food up on an electric hotplate and not on an open flame, for doing so is forbidden on Shabbat, as we have discussed in the past.
There are some other laws related to warming up food on Shabbat, for instance, that it is only permissible to reheat dry foods on Shabbat, such as rice and potatoes. However, it is forbidden to reheat liquid foods, such as soup and the like. There are several differences between Sephardic and Ashkenazi Jews regarding these laws, but we cannot delve into all of them at this point.
Bread Which Was Not Fully Baked
Based on the above, if one baked Challah and notices that they are not fully baked, usually indicated by a very doughy texture in the middle of the loaf, it is a Torah prohibition to warm these Challot on the hotplate, for by doing so, they will continue to bake.
A Food Which Is Edible As Is But Was Not Cooked
Similarly, a food which can be eaten as is, such as a tomato, and was not cooked before Shabbat may not be heated on an electric hotplate on Shabbat, for this food was never cooked and as a result of being placed on the hotplate, it will now get cooked. Thus, this constitutes a Torah prohibition, even though the tomato is perfectly edible raw.
Those who prepare store-bought couscous by pouring boiling water from a Keli Rishon (and sometimes even from a Keli Sheni) directly onto the couscous pellets may not warm up the couscous on the hotplate on Shabbat, for it is not considered cooked since it was never cooked directly on the fire. The fact that boiling water was poured over it does not make it considered fully cooked according to Halacha. Even though the couscous looks the same when placed on the hotplate to warm up and there is no apparent change in its appearance or texture, this nevertheless constitutes the prohibition of cooking on Shabbat.
If one makes mashed potatoes before Shabbat and after mashing them adds various kinds of spices, one may not warm up this dish on the hotplate since these spices were never cooked in the first place and this will constitute an actual prohibition.