Anyone who studies the laws of Shabbat, is usually mystified by the complexity of everything involved with cooking. But what is complex is not actually the halachah, but the numerous svarot (theoretical constructs) of some later rabbis, as well as the difficulty of comparing modern cooking methods and their haalchic status, with ancient ones. Rule number one is the clear statement in all classical sources "Ein Biushul Ahar Bishul" (there is no such thing as cooking after cooking); that is, once an item is fully cooked, further cooking is of no halachic significance in terms of Shabbat. Most Yemenites go by this rule even today, with Sepharadim following not far behind. If an item was cooked prior to Shabbat, no problem to stick it in the fridge, and heat it up on Shabbat. If something was only partially cooked, that's where the problems come in. It may cook further. If stirred, an uncooked part may be moved closer to the heat, and cook faster. Aside from the above rule of "Ein Bishul...", there are a number of of rabbinic considerations. Most stoves in Talmudic times were simply boxes with fuel (charcoal, twigs, etc.), with a hole for the pot to sit in. When keeping food warm, there was concern that we are actually cooking it, or appear to do so (Meihazi K'mevashel). Therefore, most of the fuel needed to be removed, with some ashes spread over the remaining coals, to show we are only keeping the food warm. I don't know about your stove, but mine doesn't work that way. In more recent centuries, this was replaced by putting a piece of sheet metal (known in Yiddish as a "blech") over the flame, with the pots sitting on the metal. In the case of the oven, it is kept at a lower than normal temperature, usually with the oven door kept slightly ajar, in order to indicate that food is merely being kept warm. (Ovens made in Israel have a special Shabbat setting. One may also use an electric hot plate (according to most). This can be kept on a timer, heating up just before the food needs to be warmed. Sepharadic Jews are only one step stricter. There is a significant minority of rabbinic opinions that the rule of "ein bishul..." applies only to solids, and not liquids. One could easily tell of a steak had been cooked or not. A pot of boiling water that, that has been cooled, would be difficult to distinguish from fresh water (although it would taste flat). Therefore, Sepharadim will cook before Shabbat anything other than liquids. An electric urn can be left on for tea or coffee, or else water may be heated on Shabbat on an overturned pan, on the blech or hot plate, so that it will not get to the level of cooking. Many, but not all, Sepharadic poskim permit heating up previously cooked dishes, so long as the majority is solid.(This is my practice). So your roast beef need not be a dried out brick. Heating in its own (previously cooked) gravy is fine. All this is for Sepharadim (although many Modern Orthodox have adopted this as well). Ashkenazim have much more complex procedures, to be discussed in my next post. But why? What are the concerns? First of all, how can you be certain that any given dish is fully cooked? Besides, what do you mean by "cooked"? Like in English, the Hebrew word has two meanings. It can include fried, roasted, broiled, as well as boiled. Or, it can mean only "boiled". So, does "Ein Bishul.."" mean only boiled foods, or all cooking methods?. Can I warm up on the blech (essentially broiling) a fully cooked boiled chicken? A Sepharadi would laugh at the question. The Ashkenazi would respond "What's your proof?" Beyond that, what do we mean by "dry"? Everything has some "juices" in it. Perhaps the "Ein bishul..." rule was only meant to be theoretical. Ashkenazim can also bring up the fact that RAMBAM interprets the "Ein Bishul.." rule to mean BIBLICALLY FORBIDDEN cooking.Also, since our stoves and ovens are adjustable, the knobs must be removed or taped into place, so as not to transgress the laws of adding fuel to a fire, or partaking in a cooking procedure.(Sephaardim dismiss this out of hand, as we have no power to make new rabbinic enactments.) So as not to transgress rabbinic law, a number of procedures bust be undertaken, essentially requiring the food to stay on the blech all of Shabbat, with brief exceptions to serve the food, carefully observing a protocol that I shall describe next time. Now, Sepharadim generally follow RAMBAM, with classical Yemenite practice even more closely following RAMBAM. But here, most Sepharadic and Yemenites have rejected his position as excessive, and contrary to older sources. Rabbi Yosef Kappah, a great Yemenite sage (whom I was privileged to know), wrote that the practice of putting cold food to be warmed was used in Yemen way before RAMBAM. Since if follows nearly all rabbinic opinions, it is justified to leave the practice in place, for liquids as well as solids. Ashkenazim have great difficulty with that approach. Details to come.