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NOTE: The laws of warming food on Shabbos are numerous and complex with a variety of opinions. This brief overview is merely a summary of the more widely accepted Ashkenazic Halachic authorities and does not serve in place of taking the time to learn this topic in a thorough fashion.
The Issue:
One may not heat/reheat even pre-cooked solid food that has cooled down either by putting it directly on the stove or in the oven. Doing so will render it prohibited to be eaten on Shabbat. In addition, food may not be left on the stovetop on Shabbos. So how then do we eat hot foods on Shabbos? Especially given that we are very particular to eat hot foods on Shabbos (see Remah OC 257:8 and Mishna Berura 257:49)!
The Solutions:
LEAVING FOOD ON A HEAT SOURCE
Any type of fully cooked food (solid or liquid) may be left on a hot plate, warming tray, 'blech' (i.e. a sheet of metal on top of a burner), in a crock pot (which has the knob covered), or in an oven (only under the conditions listed below) as long as they are placed there before Shabbos begins.
WARMING 'SOLID' FOOD ON SHABBOS
Fully cooked 'solid' foods (even entirely cold items e.g. from the refrigerator) may be placed in the following places at any time on Shabbos:
Any hot plate or warming tray (if it has a knob to adjust the temperature it must be covered or removed).
A blech on top of a flame with another tin/tray flipped upside down on top of it.
WARMING LIQUIDS ON SHABBOS
Cold liquids (even when previously cooked or heated up) may not be heated/reheated on Shabbos under any circumstances (Shulchan Aruch OC 318:4).
In regard to what is considered cold, Ashkenazim follow the opinion that only once the liquid has cooled off to the point that one normally would therefore not consume it as such, then one may not reheat it. Otherwise, if it is still warm, one may reheat it (Remah OC 318:15 as explained by the Chazon Ish, OC 37:13; and Igros Moshe, OC 4:74:2). Sefardim though follow the stringent opinion if a cooked liquid has cooled down to below Yad Soledet Bo, i,e, below 159 °F (71 °C), it is no longer considered cooked, and it is prohibited by Torah law to reheat it (Shulchan Aruch OC 318:4).
WHAT IS CONSIDERED A SOLID?
A solid is defined as any item which is not fluid and does not contain a significant amount of liquid in it (as indicated in Shulchan Aruch 318:15, ruled in Mishna Berura OC 318:32, and is the view of Rav S.Z. Auerbach and Rav Elyashiv as cited in Meor HaShabbos Vol. II, p. 49). For example, one may not reheat soup, meatballs in sauce, or brisket/chicken in gravy. However, one may take the brisket/chicken out of the gravy and reheat it (even if there still remains a small amount of liquid on the meat itself), as long there is no gravy in the pan.
WHAT ABOUT CONGEALED LIQUIDS?
If food is congealed but will liquefy when reheated (e.g. mushroom gravy), according to Ashkenazic practice one should ideally not heat it because some maintain that it's forbidden to cause food to change from liquid to solid or the reverse, as this is like producing something new, i.e. Nolad (Remah OC 318:16 as explained in Mishna Berura 318:105).
Nonetheless, a congealed liquid is considered a solid food, meaning that for Sefardim (who follow the Shulchan Aruch who has no issue with Nolad) it is entirely permitted to heat it up (Mishna Berura 318:100; Kaf Hachayim §158). In addition, all agree that if the gravy isn't substantial, one may heat the solid food together with the congealed sauce or gravy which will liquefy (Mishna Berura 318:105).
WHAT IF I AM A GUEST?
If an Ashkenazi is hosted by a Sefardi (or even a Yemenite who may be even more lenient with liquids) they may eat foods that their host heated on Shabbat. This is because the host is following the Halacha based on the accepted practice of his community (Mishna Berura 318:2.). However, one may not ask another person to do something more lenient on their behalf.
CAN I USE AN OVEN TO HEAT/REHEAT FOOD?
No, an oven may never be used to heat/reheat food on Shabbos!
BUT MY OVEN HAS SABBATH MODE!?
Yes, the Star-K Kashrus organization certifies a number of ovens that have a 'Sabbath Mode' function. However, the name 'Sabbath Mode' is very misleading (in my brief experience here in the US!), as the name implies that these types of ovens may be used without any restrictions on Shabbos. This is not correct.
Below are the guidelines for using an oven on Shabbos, whether in Sabbath Mode or not, as clarified by the Star-K themselves (summarized from SOURCE):
The food must be fully cooked and placed in the oven before Shabbos begins.
One must make certain that the oven light will not turn on/off when opening/closing the door ('Sabbath Mode' may help with this).
The oven may be opened only one time on Shabbos in order to remove all of the food.
Food may not be placed in any oven once Shabbos has begun.
If the oven is left on (including if using 'Time Bake' on a 'Sabbath Mode' oven), one must also make sure to cover the oven controls with something like aluminum foil.
IF YOU HAVE ANY QUESTIONS, PLEASE DON'T HESITATE TO CONTACT ME