We have already learned that there is no prohibition of cooking something that has already been cooked. Therefore, one may reheat cooked food on Shabbat. For example, one may remove cooked fish or fried schnitzel from the refrigerator and reheat them to yad soledet bo, as long as it does not resemble cooking (see section 18).
But there is a fundamental disagreement among the Rishonim whether this principle applies to liquids like soups and sauces. For this purpose, a liquid is something that, if left in a bowl, would spread out evenly. If it retains its shape, it is considered a solid. According to Rambam, Rashba, and Ran, the principle of ein bishul aĥar bishul applies to cooked soup as well. Therefore, based on their opinion, one may take soup from the refrigerator on Shabbat and heat it, as long as it does not look like he is cooking (section 18 below).
However, according to Rashi, Rosh, Smag, and Smak, the principle of ein bishul aĥar bishul applies only to solid food, and one may reheat them on Shabbat. Liquids, on the other hand, even if they had been fully cooked, may not be reheated once they have cooled off, and doing so is considered Bishul. This is because when it comes to solid foods, the primary purpose of cooking them is to create the cooked or baked taste. Once bread is baked, the difference between eating it hot or cold is insignificant. The same is true of fish and potatoes; it makes little difference whether they are served hot or cold. Therefore, from the moment that these foods acquire a cooked or baked taste, their cooking is considered complete. Accordingly, even if they have cooled off, one may reheat them, because cooking no longer affects them significantly. But for liquids, their temperature is an integral part of defining them as cooked. There is a fundamental difference between cold soup and hot soup, and the same is true of tea and coffee as well. Therefore, for liquids, we say yesh bishul aĥar bishul (there is a prohibition of cooking something that has already been cooked). Thus if a liquid has cooled down, one who reheats it to yad soledet bo transgresses the Torah prohibition of cooking.
Shulĥan Arukh rules in accordance with the stringent opinion (318:4), and Sephardim follow this ruling. Thus, for Sephardim, if a cooked liquid has cooled down to below yad soledet bo, it is no longer considered cooked, and it is prohibited by Torah law to reheat it past yad soledet bo. Therefore, Sephardim may not take soup out of the refrigerator and heat it. Similarly, if they remove soup from the plata, and it cools off to below yad soledet bo, they may not replace it on the plata. If it is known with certainty that the liquid is still yad soledet bo, it may be replaced (as long as it meets the conditions explained below in section 19). If one had situated it near the edge of the plata (which tends to be cooler), he may move it to the center and thus heat it more effectively.
If most of the food in a cold pot is solid, but the pot also has liquid sauce or gravy, one may not heat the pot. This is because by doing so one will also be heating up the liquid, which would constitute Bishul. The way around this is to remove the solid food and heat it by itself. One does not need to worry about the little moisture remaining upon it. Similarly, one may reheat cold food like meat or fish, even if there is a bit of wetness on them, because that liquid is not important and is batel (rendered insignificant) with respect to the solid food.
If food is congealed but will liquefy when reheated, like mushroom gravy, it is considered a solid food, and one may heat it (MB 318:100; Kaf Ha-ĥayim §158). However, according to Ashkenazic practice, le-khatĥila one should not heat it because some maintain that le-khatĥila one should not take an action that will cause food to change from liquid to solid or the reverse, because this is like producing something new (see 12:12 below). However, if the gravy is secondary to the solid food, then even Ashkenazic custom would permit le-khatĥila heating the food together with the congealed sauce or gravy, which will liquefy.